The Transit Light Curve Project. XI. Submillimagnitude Photometry of Two Transits of the Bloated Planet WASP-4b

We present photometry of two transits of the giant planet WASP-4b with a photometric precision of 400-800 parts per million and a time sampling of 25-40 seconds. The two midtransit times are determined to within 6 seconds. Together with previously published times, the data are consistent with a constant orbital period, giving no compelling evidence for period variations that would be produced by a satellite or additional planets. Analysis of the new photometry, in combination with stellar-evolutionary modeling, gives a planetary mass and radius of 1.237 +/- 0.064 M_jup and 1.365 +/- 0.021 R_jup. The planet is 15% larger than expected based on previously published models of solar-composition giant planets. With data of the quality presented here, the detection of transits of a "super-Earth" of radius 1.75 R_earth would have been possible.