Spectroscopic observations of the interacting binary BY Crucis.

We present multi-wavelength spectroscopic data of the 106 day period interacting binary BY Crucis (Daems et al. 1997).ThesystemconsistofanearlyF-typesupergiantthatlls itsRochelobeandamoremassivecompanion,hiddeninathick accretion disc. In the far-UV spectra (from 1200 A to 1950 A), strong emission lines of highly ionised elements are observed, super- posed on a continuum. Both components of the UV flux origi- nate from the accretion disc around the companion. In the UV range from 1950 A to 3150 A the radiation of the F-type mass donor is dominant. Inthevisualandtheredrangewefocusedourinterestonthe Balmerlines(H,H andH),He I5876 A,Na I5890-5896 A, andtheforbiddenline(O I)6300 A.Wealsoobtainedspectrain variousotherregions,rangingfrom3900 A to6700 A.Evidence of mass transfer is observed in the He I-line, the Na I-lines, H and some Fe-lines. The H-line shows no variability, and its double peaked prole implies that it is formed in a circumbi- nary disc or envelope. The (O I) line conrms the presence of matter around the binary system. By analysing the line prole variability of He I, we construct a tentative image of the gas flow in the accretion disc around the unseen companion. The Balmer lines are used to probe the outer, cooler gas, flowing through the outer Lagrangian point into a circumstellar disc or envelope.