THE FIRST PHOTOMETRIC ANALYSIS OF THE OVERCONTACT BINARY MQ UMa WITH AN ADDITIONAL COMPONENT

The first V R-c I-c band light curves of MQ UMa are presented and analyzed using the Wilson-Devinney (W-D) program. It is discovered that MQ UMa is an A-subtype contact binary with a high fill-out (f = 82%) and a low mass ratio (q = 0.195), which indicates that it is in the late evolutionary stage of late-type tidal-locked binary stars. The mass of the primary and secondary stars is estimated and the evolutionary status of the two components are placed on the H-R diagram. The W-D solutions also indicate that MQ UMa may be a triple system with an additional F5V type component. A sample of 16 high fill-out, low mass ratio overcontact binaries is collected and their possible evolution scenarios are discussed. Using the five times of minimum light recently observed together with those collected from the literature, the authors find that the observed-calculated (O-C) curve exhibits cyclic period variation. The cyclic period change also reveals the presence of a tertiary component, which may play an important role in the formation and evolution of this binary system by drawing angular momentum from the central system.