Light-Curve and Radial Velocity Study of the Contact Binary BD +42 2782

BD +42 2782 was recently discovered to be a variable star with a W UMa-type, eclipsing-binary light curve. We have obtained the first photoelectric light curves (R_C, I_C) and also the first radial velocity curves for this binary. These show the system to be a W UMa binary of the W type, with the less massive component eclipsed at primary minimum. The light and velocity curves have been analyzed in a consistent manner using the Wilson-Devinney synthetic light-curve code. The binary has an orbital inclination of 74°, a mass ratio of 0.48, a secondary component ~150 K hotter than the primary, and an overcontact fill-out factor of 34%. Absolute parameters of the component stars are M₁=1.38 M_solar, R₁=1.29 R_solar, L₁=1.75 L_solar, and M₂=0.67 M_solar, R₂=0.95 R_solar, L₂=1.05 L_solar, and the calculated distance is 124+/-15 pc. Even when corrected for energy transfer between the two components, the more massive component is found to be underluminous for its mass. Several means to explain this are explored, but no completely satisfactory explanation is found.