2024MNRAS.529.3113W

We presented the multifilter light curves of CSS_J154915.7+375506 inaugurally, which were observed by the 1.5-m Astronomicheskiy Zerkalniy Telescope 22 ( AZT-22) telescope at Maidanak Astronomical Observatory. A low-resolution spectrum obtained by Large-sky-Area Multi-Object fiber Spectroscopic Telescope reveals that it is an A-type (spectral A type) close binary. By analysing the BVRI total-eclipse light curves, we are able to derive a reliable photometric solution for this system, which indicates that CSS_J154915.7+375506 is an extremely low mass ratio (q = 0.138) marginal contact binary system. The location in the Hertzsprung-Russell (HR) diagram shows that its secondary component with a much smaller mass is the more evolved one, indicating that the mass ratio reversal occurred. The present secondary component had transferred a significant amount of mass to the present primary one. By the combination of a total of 20 times of minimum, we investigated its O-C (observed minus calculated) curve. A periodic oscillation and a possible period decrease have been detected. As the period decreases, the system will evolve towards the contact phase. This makes CSS_J154915.7+375506 a valuable case to study the formation scenario of contact binaries through mass reversal. The periodic oscillation suggested a third body with a minimal mass of $0.91\, {\rm M}_{\odot }$, which is larger than that of the less massive component in the central binary. This implies that the secondary body was not replaced by the third body during early stellar interactions, indicating that it is a fossil system and retains its original dynamical information.