THE STRONGLY CONSTRAINED INTERACTING BINARY BY CRUCIS

We discuss spectroscopic and photometric observations (UV to IR) of BY Crucis, a new bright member of the interacting binaries of the W Serp entis class. The orbital period is 106.4 days, and the mass function a mounts to 5.92 M. The primary is an early-F supergiant which fills its Roche lobe; the massive secondary is hidden inside a thick accretion disk, which is probably the dominant light source in the ultraviolet. The observed variations in the photometric lightcurves are caused main ly by ellipsoidal variations. It is unlikely that an eclipse occurs. A particularly interesting circumstance is that BY Cru is a probable me mber of a visual multiple system which contains another evolved star. This circumstance enables us to estimate with some accuracy the age of the interacting binary and the initial mass of the primary. The histo ry of this binary is then much more constrained than for the other W S erpentis stars. Also taking into account the fact that the binary has avoided Case C Roche lobe overflow, we determine upper and lower bound s for the initial and present masses of both components, and conclude that mass transfer has only been moderately nonconservative.