THE ACTIVE CONTACT BINARY TY UMA REVISITED: IS IT A QUADRUPLE STAR?

TY UMa is an F-type eclipsing binary star. Four-color light curves and radial velocities of this system were presented and simultaneously analyzed using the W-D code. It is found that TY UMa is a W-subtype shallow contact binary system (f = 13.4%) with a mass ratio of q = 2.523. In order to explain the asymmetric light curve of this binary, a dark spot on the less massive component was employed. Our newly determined 31 times of minimum light, including those collected from the literature, have been used to analyze orbital period changes of TY UMa. The complicated period variation could be sorted into a secular period increase at a rate of dp/dt = +5.18(+/- 0.21) x 10(-7) days yr(-1), a 51.7 yr periodic modulation (A(3) = 0.0182 days), and a very small amplitude cyclic oscillation with a period of 10.0 yr (A(4) = 0.0015 days). The long-term increase of the period can be explained by mass transfer from the less massive component to the more massive one. The Applegate mechanism may impossibly explain the two cyclic components in the period. The two cyclic variations are very likely caused by the light travel time effect of third and fourth components, suggesting that TY UMa is a quadruple system.