1997A&A...327..640D

We present multi-wavelength spectroscopic data of the 106 day period interacting binary BY Crucis (Daems et al., 1997A&A...317..823D). The system consist of an early F-type supergiant that fills its Roche lobe and a more massive companion, hidden in a thick accretion disc. In the far-UV spectra (from 1200Å to 1950Å), strong emission lines of highly ionised elements are observed, superposed on a continuum. Both components of the UV flux originate from the accretion disc around the companion. In the UV range from 1950Å to 3150Å the radiation of the F-type mass donor is dominant. In the visual and the red range we focused our interest on the Balmer lines (H_α, H_β and H_δ), He I 5876Å, Na I 5890-5896Å, and the forbidden line [O I] 6300Å. We also obtained spectra in various other regions, ranging from 3900Å to 6700Å. Evidence of mass transfer is observed in the He I-line, the Na I-lines, H_β and some Fe-lines. The H_α-line shows no variability, and its double peaked profile implies that it is formed in a circumbinary disc or envelope. The [O I] line confirms the presence of matter around the binary system. By analysing the line profile 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.