2016AJ....151...67Z

II UMa is a late F-type (F5) contact binary with a close-in tertiary and a distant visual companion. According to the four-color (B V R_cI_c) light curves' solutions of II UMa, it is a high fill-out (f = 86.6%) and low-mass ratio (q = 0.172) contact binary system, which indicates that it is at the late evolutionary stage of late-type tidal-locked binary stars. The masses of the primary star and secondary star are calculated to be M₁ = 1.99 M_☉ and M₂ = 0.34 M_☉. The primary star has evolved from the zero-age main sequence, but it still appeared before the terminal-age main sequence, and the secondary star is even more evolved. Considering the mass ratio (M₃/M₁ = 0.67) obtained by spectroscopic observations, the mass of the close-in tertiary is estimated to be M₃ = 1.34M_☉. The period variations of the binary system are investigated for the first time. According to the observed-calculated (O-C) curve analysis, a continuous period increase at a rate of dP/dt = 4.88 x 10⁷ day/yr is determined. The parabolic variation in the O-C curve may be part of a cyclic period of change, or the combined period of change of a parabolic variation and a cyclic one. More instances of minimum light are needed to confirm this. The presence of the tertiary component may play an important role in the formation and evolution of this binary system by drawing angular momentum from the central system during the pre-contact stage.