LO And: an A-subtype contact binary with a very cool third component

Abstract

Despite the intensive investigations since the discovery of LO And approximately 60 yr ago, its evolutionary status and subtype are still a matter of controversy. By simultaneously modeling the radial-velocity curves and new light curves with the Wilson-Devinney code, we present new geometric, photometric and absolute parameters for this system. The simultaneous solution suggests that LO And is an A-subtype contact binary with a contact degree of 32.4%. The absolute parameters are modified to become M 1 = 1.409 M ⊙, M 2 = 0.449 M ⊙, R 1 = 1.36 R ⊙ and R 2 = 0.83 R ⊙. From our observations and data from surveys, we determined 334 eclipse timings. The O – C diagram, constructed from the new eclipse timings and those reported in the literature, reveals a secular increase and a cyclic variation in its orbital period. The former is caused by conservative mass transfer from the secondary component with less mass to the primary one with more mass. The latter may be explained by either the cyclic magnetic activity on the two components or the light-time effect due to a third body. With the absolute physical parameters, we investigated its evolutionary status, and find that LO And is an unevolved contact binary undergoing thermal relaxation oscillation, which will eventually coalesce into a single star with rapid rotation.