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II UMa is a late F-type (F5) contact binary with a close-in tertiary and adistant visual companion. According to the four-color ($B$ $V$ $R_c$ $I_c$)light curves' solutions of II UMa, it is a high fill-out (f=$86.6\,\%$) and lowmass ratio ($q = 0.172$) contact binary system, which indicate that it is atthe late evolutionary stage of late-type tidal-locked binary stars. The mass ofthe primary star and secondary one are calculated to be $M_1 = 1.99M_\odot$,$M_2 = 0.34M_\odot$. The primary star has evolved from ZAMS, but it is stillbefore TAMS, and the secondary star is even more evolved. Considering the massratio ($M_3/M_1 = 0.67$) obtained by spectroscopic observations, the mass ofthe close-in tertiary is estimated to be $M_3 = 1.34M_\odot$. The periodvariations of the binary system is investigated for the first time. Accordingto the observed-calculated ($O$-$C$) curve analysis, a continuous periodincrease at a rate of $dP/dt=4.88\times{10^{-7}}day\cdot year^{-1}$ isdetermined. It may be just a part of a cyclic period change, or thecombinational period change of a parabolic variation and a cyclic one. Moretimes of minimum light are needed to confirm this. The presence of the tertiarycomponent may play an important role in the formation and evolution of thisbinary system by drawing angular momentum from the central system during thepre-contact stage.

Photometric and period investigation of the late F-type overcontact binary II UMa