Absolute dimensions of eclipsing binaries. XXIV. The Be star system DW Carinae, a member of the open cluster Collinder 228

DW Carinae is a close but detached early B-type eclipsing binary in the young open cluster Cr 228. We have measured accurate physical properties of its components (masses and radii to 1%) and used these to derive the age, metallicity and distance of Cr 228. The rotational velocities of both components of DW Car are high, so we have investigated the performance of double-Gaussian fitting, one- and two-dimensional cross-correlation and spectral disentangling for measuring radial velocities in the presence of strong line blending. Gaussian and cross-correlation analyses require substantial corrections for the effects of line blending, which are only partially successful for cross-correlation. Disentangling is to be preferred because it does not assume anything about the shapes of spectral lines, and is not significantly affected by blending. Complete Stromgren uvby light curves have been obtained and accurate radii have been measured from them using the Wilson-Devinney program, constrained by an accurate spectroscopic light ratio. The Teffs and reddening have been found from Stromgren photometric calibrations. The masses and radii of DW Car are M_A = 11.34 +/- 0.12 Msun, R_A = 4.558 +/- 0.045 Rsun, M_B = 10.63 +/- 0.14 Msun and R_B = 4.297 +/- 0.055 Rsun. The Teffs are Teff_A = 27900 +/- 1000 K and Teff_B = 26500 +/- 1000 K, and the reddening is E(b-y) = 0.18 +/- 0.02 mag. Using empirical bolometric corrections we find a distance modulus of 12.24 +/- 0.12 mag for DW Car, which is in agreement with, and more accurate than, literature values for Cr 228. A comparison between the properties of DW Car and theoretical predictions is undertaken in the mass--radius and mass--Teff diagrams. Good agreement is found for an age of about 6 Myr and a metal abundance of Z approx 0.01 (abridged).