PROPERTIES OF THE CLOSE-IN TERTIARY IN THE QUADRUPLE SYSTEM V401 CYG

Abstract

V401 Cyg is a quadruple system in which the spectroscopic signature of a close-in tertiary and a distant visual companion star were reported. Orbital properties of the close-in companion should provide valuable information on the formation of close binaries and stellar dynamical interaction. By analyzing new times of minimum light together with those collected from the literature, we discovered that the observed−calculated (O − C) curve of V401 Cyg shows a cyclic change with a short period of 3.5 yr and a semi-amplitude of 0.00436 days while it undergoes an upward parabolic variation. Those photoelectric and CCD data covered more than two cycles and were analyzed for the light-travel time effect via the presence of the tertiary companion. The mass of the third body was determined to be M3sin i' = 0.65(± 0.08) M☉, which is close to the value estimated from the spectroscopic data (M3 ∼ 0.64 M☉). This reveals that the orbital inclination of the tertiary was about i' ∼ 90°, indicating that the contact components of V401 Cyg have the possibility of being eclipsed by the tertiary at an orbital distance of about 3.0 AU, and it may be a triply eclipsing hierarchical triple system. The upward parabolic change indicates a period increase at a rate of revealing a mass transfer from the secondary to the primary ( M☉ yr−1). This is consistent with the predictions of the theory of thermal relaxation oscillation (TRO) suggesting that V401 Cyg is undergoing an expanding-orbit stage in the TRO cycles.