1997A&A...317..823D

We discuss spectroscopic and photometric observations (UV to IR) of BY Crucis, a new bright member of the interacting binaries of the W Serpentis class. The orbital period is 106.4days, and the mass function amounts to 5.92M_☉. 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 mainly by ellipsoidal variations. It is unlikely that an eclipse occurs. A particularly interesting circumstance is that BY Cru is a probable member 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 history of this binary is then much more constrained than for the other W Serpentis stars. Also taking into account the fact that the binary has avoided Case C Roche lobe overflow, we determine upper and lower bounds for the initial and present masses of both components, and conclude that mass transfer has only been moderately non-conservative.