2006AJ....131.1032Q

BO CVn and SS Com are two short-period W UMa-type binary stars with spectral types earlier than F5. In the present paper new CCD photometric light curves in B, V, and R bands of BO CVn and the first complete light curve in V band of SS Com are presented. The light curves of the two binaries are symmetric, and no variations of the light curves were found for both systems that are quite unlike those of cooler overcontact binaries. These observational properties may suggest weak photospheric dark spot activity in the two systems during the observational time intervals. Photometric solutions of the two W UMa-type binaries were derived by using the 2003 version of the Wilson-Devinney (W-D) method. The new solutions suggest that they are overcontact binary stars with degrees of overcontact of f=40.6% for BO CVn and f=49.6% for SS Com. Our new times of light minimum confirmed the long-time period increases of the two systems, and the rates of continuous period increase were revised. The long-time period increases, the orbital periods, the degrees of overcontact, and the mass ratios all suggest that they are on an evolutionary stage of mass transfer from the less massive component to the more massive one, and SS Com will reach the present evolutionary state of BO CVn. The slow evolution toward extreme mass ratio will cause them finally to coalesce to single stars. Therefore, both of them are AW UMa-type near-coalescent overcontact binary systems.