RZ Pyx: A special short period detached massive binary with two cool stellar companions in a quadruple system

Abstract

RZ Pyx is one of a small group of short-period B-type eclipsing binaries with an orbital period of 0.656 days. Several new CCD times of light minimum of RZ Pyx were obtained. Together with all available photoelectric and CCD times of light minimum, the changes of the orbital period are investigated for the first time. Meanwhile, previously published light curves are reanalyzed with the Wilson–Devinney code. Based on the analysis of the O–C diagram, two cyclic variations with periods of 37.1 years and 9.7 years are discovered superimposed on a continuous increase at a rate of dP/dt=+0.32×10−7 day yr−1. The light curve solutions suggest that RZ Pyx is a marginal detached binary system where both components do not overfill their respective Roche lobes. The fill-out factors of the primary and the secondary component are 95.5(±0.8)% and 99.1(±1.9)%, respectively, revealing that the secondary is nearly filling its Roche lobe. This may indicate that RZ Pyx has undergone a mass-transferring evolutionary stage and it is on the marginal detached stage temporarily. The long-term increase in the orbital period could be explained by the enhanced mass loss by stellar winds of the two detached massive components. Since the two binary components are early-type stars, the two cyclic oscillations could be plausibly interpreted as the results of the light travel-time effect caused by the presence of two additional companions. It is estimated that the masses of the two additional bodies are no less than 0.36M⊙ and 0.21M⊙, respectively. The two cool stellar companions are orbiting the central binary at orbital separations of 23.1 and 9.5 au in a quadruple stellar system. Both the marginal detached configuration and the presence of two cool stellar companions make RZ Pyx a very interesting binary system for further investigations.