Astronomy and astrophysics vol:317 issue:3 pages:823-831
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 106.4 days, and the mass function amounts 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 mainly by ellipsoidal variations. It is unlikely that an eclipse occurs.