Spectroscopic orbit and variability of the Be star V1624 Cyg = 28 Cyg

H.Ak; H. Bo\v{z}i\'c; J. Labadie-Bartz; J. \v{S}vr\v{c}kov\'a; M. Zummer; P. Dole\v{z}al; P. Harmanec; P. Koubsk\'y; P. Zasche; S. Ranguin

REVIEW 4 major objections 5 minor 39 references

Spectroscopic orbit and variability of the Be star V1624 Cyg = 28 Cyg

T0 review · 4 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read H-alpha emission-wing radial velocities show that the Be star V1624 Cyg = 28 Cyg moves on the 359.26-day interferometric orbit; adding IUE secondary velocities yields masses of about 5.6 and 0.66 solar masses.

desk verdict The 359-day orbit is plausible, but the amplitude K1 is fragile and the mass ratio rests on marginal IUE data; still worth a serious referee. read the letter →

arxiv 2506.06722 v1 pith:QDVSYYFN submitted 2025-06-07 astro-ph.SR

P. Harmanec , S. Yang , P. Koubský , J. Labadie-Bartz , P. Doležal , S. Ranguin , H. Božić , J. Švrčková

show 3 more authors

M. Zummer P. Zasche H.Ak

This is my paper

classification astro-ph.SR

keywords Bestarsspectroscopicbinariesradialvelocitiesemission-lineV1624Cyg28long-periodcompactcompanions

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper sets out to show that the bright Be star V1624 Cyg = 28 Cyg is a genuine binary with a 359.26-day orbital period, using radial velocities measured on the steep wings of its H-alpha emission line. Those velocities follow the same orbit that interferometry recently found, so the paper would turn an astrometric detection into a confirmed spectroscopic orbit for the Be primary. When the primary's orbit is combined with 25 archival ultraviolet velocities of the possible companion, the preliminary solution gives masses of about 5.6 and 0.66 solar masses, the smaller value being a low-mass compact object. The paper also assembles photometry and spectroscopy spanning decades to show that the star's emission, brightness, and colour vary strongly but the circumstellar disc never completely disappears. If correct, V1624 Cyg becomes one of the longest-period confirmed Be binaries with a post-mass-transfer companion, giving the duplicity picture of the Be phenomenon a concrete new data point.

What carries the argument

The load-bearing measurement is the radial velocity of the steep wings of the double H-alpha emission, obtained by the mirror method of comparing the direct and flipped line profiles on screen. Emission wings are used because, as argued with reference to Ruždjak et al. (2009), they trace the orbital motion of the Be primary more faithfully than the absorption core, which is disturbed by rapid line-profile and V/R changes. The period search relies on Deeming amplitude periodograms, and the orbital fits are made with the FOTEL program; in Solution 1 the period and epoch are fixed to the interferometric values, and separate systemic velocities are allowed for the CCD, photographic, and IUE velocity sets to absorb zero-point offsets.

What would settle it

Take high-resolution far-ultraviolet spectra of V1624 Cyg at the phases where the 359.26-day ephemeris predicts the companion at maximum and minimum radial velocity; if the companion's lines do not appear, or do not shift by roughly $43\,\mathrm{km\,s^{-1}}$ in antiphase with the primary, then the mass ratio and the two derived masses are wrong even if the primary's orbit is real.

Watch

Extended reading notes

Core claim

On its own terms, the central discovery is that the H-$\alpha$ emission-wing radial velocities of V1624 Cyg's Be primary reproduce the 359.26-day circular orbit derived from interferometry: a period search over modern and historical emission-wing RVs peaks near 355-360 days, and an orbital solution with the interferometric period fixed gives a primary semi-amplitude $K_1 = 5.10 \pm 0.88\,\mathrm{km\,s^{-1}}$. Adding the 25 IUE secondary RVs fixes the amplitude ratio $K_1/K_2 = 0.118 \pm 0.033$, and with the interferometric inclination of $61.3^\circ$ the masses become $m_1 \approx 5.6\,M_\odot$ and $m_2 \approx 0.66\,M_\odot$. The paper presents this as a preliminary solution, noting that the electronic spectra do not cover the radial-velocity minimum and that the secondary detections were only marginal.

Load-bearing premise

The mass ratio and the derived masses rest on the assumption that the 25 ultraviolet radial velocities reconstructed by Wang et al. (2018) really trace the companion's orbital motion, even though those detections were marginal and later HST/STIS spectra did not show the companion.

Editorial extensions

If this is right

V1624 Cyg becomes a confirmed member of the small group of Be binaries with near-one-year periods and compact companions, adding direct support to the idea that duplicity is tied to the Be phenomenon.
With the assumed inclination, the primary mass of about $5.6\,M_\odot$ is consistent with a B3V classification, and the orbital and rotational axes agree within their errors.
The compiled light and spectrum history shows that the double H-alpha emission never fully vanished, so the circumstellar disc has persisted over the entire century of observations.
The small primary semi-amplitude of about $5\,\mathrm{km\,s^{-1}}$ and the near-year period mean that similar long-period Be binaries could easily have been missed, so long time series of H-alpha spectra are the way to find them.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

Beyond the paper, the near-year value of the period makes an observing-season alias a real alternative worth testing, so a dedicated H-alpha campaign that samples the radial-velocity minimum would be the cleanest check of the orbit.
The Hubble non-detection may mean the companion is intrinsically fainter or more variable in the far ultraviolet than during the IUE epoch, so future searches should be timed to predicted velocity extrema rather than assumed to work at any phase.
The same emission-wing technique could be applied to other bright Be stars with suspected compact companions found by astrometry, offering a way to confirm long-period post-mass-transfer binaries without waiting for a secondary detection.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

The genuinely new thing here is the independent period search: the emission-wing RVs produce a periodogram peak near 359 days without being fed the interferometric period, and a free-period solution gives 358.98 ± 0.27 d, consistent with Klement et al. (2024). That is a real step forward, and the long-term variability documentation (Hα equivalent widths, UBV photometry) is useful and carefully compiled. The authors are also honest about their data: they explicitly say the CCD RVs alone do not cover the RV minimum and give a lower semi-amplitude, and they note the HST non-detection of the secondary is unexplained. Credit where due: they do not oversell the orbit as final.

Now the soft spots. The load-bearing one is K1. It is 5.10 km/s from the adopted solution, but the only data covering the RV minimum are the Ann Arbor photographic plates from 1932, whose rms scatter is 15.75 km/s — three times the fitted amplitude. Twelve outliers were removed post hoc, and while the authors give the list and a plausible reason (confusion of emission peaks), that is a lot of weight on century-old data. The periodogram is shown but no false-alarm probability is reported, so the 359-day peak could be a sidelobe of the 236-day peak or a window artifact for all we know. The mass ratio and the 0.66 Msun compact companion rest on 25 IUE RVs that Wang et al. (2018) themselves called marginal and that HST/STIS could not confirm. The paper's own text admits this in Sect. 6. None of these are fatal if the paper is read as a preliminary confirmation, but the abstract's 5.6 and 0.66 Msun are presented without the necessary caveat that both K1 and K2 are fragile.

A minor point: the paper would be stronger with a Monte Carlo or bootstrap uncertainty on the masses that propagates the rms of the photographic RVs and the IUE scatter, rather than the formal FOTEL errors. The data tables are at CDS, which is good, but no code is released.

Who is this for? People working on the Be binary population, especially the duplicity–Be connection. It is a useful data point but not a final one. I would send it to a serious referee, with the instruction to focus on the periodogram significance and the robustness of K1 to exclusion cuts. If those hold, the paper is publishable as a preliminary orbit; if they do not, the abstract needs to be significantly softened.

Referee Report

4 major / 5 minor

Summary. This paper combines newly measured H-alpha emission-wing radial velocities from electronic spectra (DAO, Ondrejov, BeSS, NRES) with archival photographic RVs (Losh 1932; Abt and Levy 1978) and published IUE secondary RVs (Wang et al. 2018) to test whether the Be star V1624 Cyg moves on the 359.26 d orbit recently proposed from interferometry. Periodograms of the emission-wing and absorption RVs show a peak near 355-360 d, and two orbital solutions are derived with FOTEL: solution 1 fixes P=359.26 d and solution 2 converges to P=358.98 +/- 0.27 d. The fits yield K1 about 5.1-5.4 km/s and K1/K2 about 0.118, which, combined with i=61.3 degrees from Klement et al. (2024), give m1 about 5.6 solar masses and m2 about 0.66 solar masses. The paper also documents long-term H-alpha equivalent-width and UBV variability showing that the star never lost its circumstellar disc.

Significance. If the spectroscopic confirmation holds, V1624 Cyg would be a valuable addition to the small sample of confirmed long-period Be binaries with post-mass-transfer companions, and the independent periodogram support for the interferometric period is a real strength of this work. The paper also makes effective use of heterogeneous archival material and clearly presents the data inventory. However, the confirmation and the mass determination are not yet on equal footing: the period is independently supported, but the orbital amplitude K1 and the mass ratio rest on century-old photographic velocities with scatter several times the amplitude and on 25 marginal IUE secondary detections. The result is therefore significant but provisional; with additional phase coverage and a significance assessment it could become a reference confirmation.

major comments (4)

[Section 5, Table 8, Fig. 8] The primary semi-amplitude K1 is the load-bearing quantity for the central claim, and the paper itself shows that it is not pinned down by the modern data. The CCD emission-wing RVs do not cover the RV minimum, and the solution based solely on them gives a significantly lower semi-amplitude; the only phase coverage of the minimum comes from the Ann Arbor photographic velocities, whose rms in solution 1 is 15.75 km/s against a fitted K1 of 5.10 km/s. Because the masses scale roughly as m1 sin^3 i proportional to (K1+K2)^2 K2, an error in K1 of 2-3 km/s shifts the component masses by tens of percent and weakens the claimed spectroscopic confirmation of the interferometric orbit. Please report a solution using only the electronic data, a solution using the electronic plus photographic data without the +40 km/s cut, and a quantitative sensitivity analysis of K1 and the masses to the choice of included data.
[Section 5, Table 7] The exclusion of 12 Ann Arbor RVs with residuals larger than 40 km/s is a post hoc cut applied after fitting the orbit, and the paper gives no independent criterion for the threshold. Several of the omitted epochs cluster around HJD 2438xxx, so the cut may preferentially remove particular orbital phases and thereby bias K1 and the phase coverage of the minimum. The statement that the elements of a solution based on all Ann Arbor spectra are 'quite similar' is not backed by a table; please provide the full-sample solution and show how P, K1, and the rms change for a range of thresholds (for example, 30, 40, and 50 km/s).
[Section 5, Fig. 7] The periodogram analysis is the main independent evidence for the 359 d period, but no false-alarm probability or significance level is reported for the Deeming amplitude periodograms. With heterogeneous velocities spanning a century, different zero points, and strong rapid line-profile variability, a peak height alone is not enough to establish significance. Please add a bootstrap or equivalent false-alarm estimate for the 359 d peak in both the emission-wing and absorption datasets, and also report the significance of the 236 d peak.
[Section 6, Table 8] The adopted masses of 5.6 and 0.66 solar masses depend on the mass ratio K1/K2 = 0.118 +/- 0.033 derived from only 25 IUE secondary RVs that the original authors described as marginal, combined with an inclination from Klement et al. (2024). Given that Wang et al. (2021) could not detect the secondary in HST/STIS spectra and that the paper offers no explanation ('It is not quite clear why'), the component masses should be flagged as provisional. Please propagate the full allowed range of K1/K2 and of i, and state explicitly how the masses change if the secondary RVs are excluded and K2 is instead fixed by the astrometric total mass.

minor comments (5)

[Throughout] The notation '359d.26', '226d.0', and '0d.7564' is nonstandard; please write '359.26 d' and similar forms consistently.
[Section 4] The sentence 'observations from the interval of of JD 2454000-55000' contains a duplicated 'of'.
[Section 5] The phrase 'absoption RVs' should be 'absorption RVs'.
[Table 8] It would help to state explicitly that K2 is derived from K1 divided by the fitted K1/K2 ratio, and that the m sin^3 i values assume the circular orbit and the adopted inclination.
[Section 6] The statement that V1624 Cyg has 'one of the longest orbital periods yet confirmed' is stronger than the preliminary nature of the solution warrants; 'candidate' would be more accurate until the K1 issue is resolved.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the 359-day spectroscopic orbit and masses are obtained from independent RV time-series analysis rather than being read back from the interferometric orbit or from the adopted secondary RVs.

full rationale

The paper's central claim—that the Be primary moves in the 359.26 d orbit found by Klement et al. (2024)—is not circular. The emission-wing RVs are analysed by an independent Deeming (1975) periodogram, which returns a best period close to 359 d before any fitting to the interferometric elements; solution 2 then allows P and T to converge freely (P = 358.98 +/- 0.27 d, T = 59509.9 +/- 5.9), so the spectroscopic orbit is not read off from the interferometric input by construction. The primary semi-amplitude K1 and mass ratio K1/K2 are fitted from the combined RV material (photographic, CCD, IUE), and the masses follow from the standard mass function with the externally adopted inclination i = 61.3 deg, rather than from any parameter that already encodes the 5.6/0.66 M_sun result. The author-overlapping citation used to justify the emission-wing method (Ruzdjak et al. 2009) is an empirical validation external to this paper's fit and is not used to forbid alternatives; the paper additionally checks the CCD/Ann Arbor systemic-velocity agreement as an internal control. The weaknesses noted in the manuscript—the historical photographic RVs having rms scatter larger than K1, and the failure of HST/STIS to confirm the IUE secondary—are data-quality and confirmation issues, not equivalences between premises and conclusions. No load-bearing circular step was found.

Assumptions & free parameters 9 free parameters · 4 assumptions · 0 invented entities

The masses and orbit depend on fitted velocity amplitudes, separate systemic zero points for three data sources, an adopted inclination from interferometry, and a hand-chosen O-C cutoff. The central orbit confirmation does not rely on invented entities; the mass estimate rests on marginal secondary RVs and an adopted inclination.

free parameters (9)

K1 (primary semi-amplitude) = 5.10 +/- 0.88 km/s (solution 1), 5.40 +/- 0.87 km/s (solution 2)
Fitted to 432 H alpha emission-wing RVs; it sets the primary velocity amplitude and directly controls the derived masses.
K1/K2 (mass ratio proxy) = 0.118 +/- 0.033 (solution 1)
Fitted using 25 IUE RVs of the secondary from Wang et al. (2018), which the paper notes were marginal detections.
Systemic velocity for CCD spectra (gamma_CCD) = -17.34 +/- 0.58 km/s (solution 1)
Separate zero-point offset for modern electronic spectra, needed because different instruments have different velocity scales.
Systemic velocity for photographic spectra (gamma_phg) = -15.2 +/- 1.4 km/s (solution 1)
Separate zero-point offset for the Ann Arbor photographic RVs.
Systemic velocity for IUE spectra (gamma_IUE) = -25.7 +/- 6.2 km/s (solution 1)
Separate zero-point offset for the marginal IUE secondary RVs.
Orbital period P (solution 2) = 358.98 +/- 0.27 d
Fitted when not fixed to the interferometric value; used to cross-check the 359.26 d period from Klement et al. (2024).
Epoch of superior conjunction = HJD 2459524.8 (fixed, solution 1); HJD 2459509.9 +/- 5.9 (solution 2)
Fitted phase zero point for the orbital solution.
Orbital inclination i = 61.3 deg
Adopted from Klement et al. (2024), not fitted here; required to convert m sin^3 i into physical masses.
O-C residual cutoff for excluded RVs = 40 km/s
Hand-chosen threshold used to omit 12 Ann Arbor photographic RVs from the final solution; a post hoc selection rule.

assumptions (4)

domain assumption Radial velocities of the H alpha emission wings trace the orbital motion of the Be primary.
Invoked in Sect. 5 to convert measured velocities into an orbital solution; the justification is prior experience cited to Ruzdjak et al. (2009), and Sect. 3 shows the velocities are affected by rapid line-profile changes.
domain assumption The 25 IUE radial velocities from Wang et al. (2018) represent the orbital motion of the compact secondary.
Used in Sect. 5 to estimate K1/K2 and the masses. Wang et al. (2018) described these as marginal detections, and Wang et al. (2021) did not detect the secondary in HST/STIS spectra.
domain assumption Orbital inclination of 61.3 deg from Klement et al. (2024) applies to the binary and is not strongly biased by the Be disc.
Adopted in Sect. 6 to convert m sin^3 i into physical masses. The paper notes consistency with the estimated rotational inclination of 69 +/- 17 deg from Zorec et al. (2016).
standard math Deeming periodogram and FOTEL fitting provide unbiased period and orbital parameter estimates for unevenly sampled data.
Standard tools cited in Sect. 5; the paper does not justify their statistical properties, but this is routine practice in binary-star analysis.

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Cite this review

Pith. "Pith review of Spectroscopic orbit and variability of the Be star V1624 Cyg = 28 Cyg." pith.science (2026). https://pith.science/paper/QDVSYYFN

@misc{pith2026250606722,
  author       = {Pith},
  title        = {Pith review of: Spectroscopic orbit and variability of the Be star V1624 Cyg = 28 Cyg},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/QDVSYYFN}},
  note         = {Machine review of arXiv:2506.06722}
}

read the original abstract

In recent years the idea, first formulated many decades ago, that the Be phenomenon could be causally related to the duplicity of Be stars, has been repeatedly reconsidered from various perspectives. It is important, therefore, to have reliable information on Be stars, which are confirmed members of binary systems. This study is devoted to V1624 Cyg = 28 Cygni, which was recently identified as a binary with a compact secondary. By measuring the radial velocities (RVs) of the wings of the H alpha emission line and using archival data and published RVs from the International Ultraviolet Explorer, we demonstrate that the Be primary moves in the 359.26 d orbit found recently from interferometry. Our preliminary radial-velocity solution leads to binary masses of 5.6, and 0.66 solar masses. Moreover, we documented large and irregular spectral, brightness, and colour changes over a time interval of several decades to show that the object never completely lost its circumstellar matter.

Figures

Figures reproduced from arXiv: 2506.06722 by the authors.

**Figure 1.** Evidence of rapid RV variations in the Hα line measured in two dense night series of spectra. RVs of the emission wings are denoted by red circles, and those of the absorption core by blue circles. The error bars of individual RVs are shown. Top: A series of DAO spectra. Bottom: A series of NRES spectra. In [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗

**Figure 2.** Evidence of rapid V/R variations in the Hα line measured in two dense night series of spectra. Top: A series of DAO spectra. Bottom: A series of NRES spectra. already noted by Losh (1932). It must also be kept in mind that the available material is somewhat heterogenous. Losh (1932) RVs are based on the mean of Balmer Hβ and Hγ RVs, the RVs by Abt & Levy (1978) are mean RVs of a number of Balmer and He I from photog… view at source ↗

**Figure 5.** Time plot of all RVs at our disposal. Absorption RVs are shown in the upper plot, RVs of Hα emission wings are shown in the bottom plot [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗

Figures from the paper (4 more)

**Figure 7.** Figure 7: Amplitude periodograms of all RVs without early data from Losh (1932). Top: Absorption RVs. Bottom: Hα emission-line wings RVs. The frequency of the orbital period of 359d .26 is 0.002783 c d−1 , its harmonics is 0.005567 c d−1 , and the frequency of the 236 d period i…

**Figure 6.** Figure 6: Recorded brightness and colour changes based on observations secured in, or transformed to, the Johnson UBV system. At the suggestion of an anonymous referee, we tried to check whether a reasonable orbital solution could be obtained for absorption RVs from a subset of…

**Figure 8.** Figure 8: Radial-velocity curve for the final solution (top panel) and the O−C residuals from it (bottom panel). RVs are emission-line RVs, the mean of Hβ and Hγ from the Ann Arbor photographic spectra by Losh (1932) (shown by pluses), and Hα emission wings for all CCD spectra (…

**Figure 9.** Figure 9: Phase diagram of Keck RVs plotted for the ephemeris TRVmax. = HJD 2455501.016(22) + 1 d .84290(12) · E. variability makes the detection of the secondary more favourable at certain epochs only. We note that the orbital period of V1624 Cyg is one of the longest orbital p…

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.

[1] [1]

Abt , H. A. & Levy , S. G. 1978, , 36, 241

work page 1978

[2] [2]

Adelman , S. J. & Lovelace , C. A. 1997, Journal of Astronomical Data, 3, 6

work page 1997

[3] [3]

1983, , 53, 319

Andrillat , Y. 1983, , 53, 319

work page 1983

[4] [4]

2018, , 610, A70

Baade , D., Pigulski , A., Rivinius , T., et al. 2018, , 610, A70

work page 2018

[5] [5]

C., Johnson , J

Becker , J. C., Johnson , J. A., Vanderburg , A., & Morton , T. D. 2015, , 217, 29

work page 2015

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1993, , 269, 343

Bossi , M., Guerrero , G., & Zanin , F. 1993, , 269, 343

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Curtiss , R. H. 1925, Popular Astronomy, 33, 537

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[8] [8]

Deeming , T. J. 1975, , 36, 137

work page 1975