The Photometric Investigation of V921 Her Using the Lunar-Based Ultraviolet Telescope of Chang'e-3 Mission

The light curve of V921 Her in ultraviolet band observed by the Lunar-based Ultraviolet Telescope (LUT) is analyzed by the Wilson-Devinney code. Our solutions conclude that V921 Her is an early type marginal contact binary system with an additional close-in component. The binary system is under poor thermal contact with a temperature difference of nearly 700 K between the two components. The close-in component contributes about 19% of the total luminosity in the triple system. Combining the radial velocity study together with our photometric solutions, the mass of the primary star and secondary one is calculated to be M-1 = 1.784 (+/- 0.055)M-circle dot, M-2 = 0.403 (+/- 0.012)M-circle dot. The evolutionary scenario of V921 Her is discussed. All times of light minimum of V921 Her available in the bibliography are taken into account and the O-C curve is analyzed for the first time. The most probable fitting results are discussed in the paper, which also confirm the existence of a third component (P-3 = 10.2 year) around the binary system. The period of V921 Her is also undergoing a continuously rapid increase at a rate of dP/dt = +2.79 x 10(-7) day.year(-1), which may be due to mass transfer from the less massive component to the more massive one.