2022RAA....22c5024X

The photometric solutions of the ultrashort period close binary V0644 Ser based on our new complete BVRI light curves are derived by the Wilson-Devininney code. The results show that V0644 Ser is a W-type shallow contact binary, and the third light was found in the R and I bands. Compared with the light curves in 2011, the amplitude variation of the secondary maximum can be explained by the temperature change from hot to cold in the local region of the star surface caused by magnetic activity or convective instability. Combined with the Gaia parallax, the absolute parameters of this system are obtained: M₁ = 0.29(4) M_☉, M₂ = 0.56(9) M_☉, R₁ = 0.48(3) R_☉, R₂ = 0.65(3) R_☉, L₁ = 0.11(2) L_☉, L₂ = 0.16(2) L_☉. To study the period variation, we fitted the SuperWASP, CSS, ASAS-SN and ZTF survey timing data to obtain light minima times spanning 14 yr. Through O - C analysis, we find that the orbital period of this system has a long-term period decrease and periodic oscillation. The long-term period decrease can be explained by the mass transfer from more-massive component to less one and angular momentum loss via magnetic stellar wind. With the period decrease, this system is evolving from the present shallow contact phase to a relatively deeper stage predicted by the thermal relaxation oscillation theory. Periodic oscillation can be explained by the light-time effect of the cool third body. This third body may play an important role in the early formation and evolution of the binary system by removing angular momentum.