We discuss ultraviolet photometry of the eclipsing binary W Ursae Majoris obtained with the Groningen instruments on the Astronomical Netherlands Satellite (ANS). Fifty measurements of this star's brightness were obtained for each of the bandpasses at 2200, 2500, and 3300 Å. This new data is significantly more precise than Rucinski's OAO-2 photometry, enabling us to draw definitive conclusions about the distribution of brightness on the surface of W UMa. The ultraviolet light curves are generally similar in shape to those for optical passbands; the amplitudes are greater in the UV, while the depths of the primary and secondary eclipses are nearly equal at both 2200 and 2500 Å. We have analyzed the (2200 − 3300) color curve to determine how much the color temperature varies over the common envelope, concluding that any such temperature variations must be small. In terms of the traditional surface-brightness parameters for gravity darkening, temperature excess of the smaller component, and bolometric albedo, we find β = 0.03 ± 0.01, X = ΔT/T = 0.000 ± 0.009, and Abol = 0.4 ± 0.4. Further, we conclude that the star is limb darkened considerably more at 2200 than at 3300 Å.
Ultraviolet Light Curves of U Geminorum and VW Hydri