PHOTOMETRIC EVIDENCE FOR WIDESPREAD SPOT ACTIVITY ON THE PRIMARY COMPONENT OF THE SHORT-PERIOD RS CVN BINARY XY UMA

A survey of published light curves, obtained since 1975, of the eclipsing RS CVn-type binary star XY UMa, coupled with results from a recent high-precision light curve, provides substantial evidence for the existence of a polar spot on the G2V primary component. The well-known and strong variability of the shape of the light curve has been explained by the presence of surface maculation, often at low latitudes, which varies on time-scales of similar to 1 month, and which may persist for longer intervals. The 20-year span of published light curves, however, shows that the entire system brightness varies by as much as 0.5 mag in the V band. No orbital phase is exempt from this longer-term variation, suggesting that the portion of the dominant primary component that is always visible - namely the polar region - is at least partly responsible for the long-term variation in system brightness. It is proposed that the surface of the primary component of XY UMa is very like that seen in the rapidly rotating single G dwarf star AB Dor, and the RS CVn binary DM UMa, namely, dominated by a polar spot but with a second belt of spot activity at lower latitudes.