Photometric study and period variation investigation of the WUMa-type binary EQUMa

Four-color light curves of EQ UMa were obtained with the 1.0 m telescope at Weihai Observatory, Shandong University. The light variability of EQ UMa is typical of a W UMa-type binary. By analyzing the four-color light curves simultaneously, we found that EQ UMa is a semi-detached binary with the less massive component filling its inner Roche lobe. The very short period and the Algol-type configuration make EQ UMa a very good target to challenge the formation and evolution theory of low-mass Algols. We investigated the orbital period change of EQ UMa by all available times of minimum light. It is shown that the orbital period of EQ UMa displays a cyclic oscillation with a period of 12.1 yr super-imposed on long-term increase with a rate of dp/dt = 1.77 x 10(-7) dyr(-1). The long-term period increase should be caused by themass transfer from the less massive component to the more massive one. The cyclic modulation is very probably produced by the light travel time effect of a third component.