The eclipsing supersoft X-ray binary CAL 87

We present and discuss 25 spectra obtained in 1996 November, covering all p hases of the CAL 87 binary system. These spectra are superior both in signa l-to-noise ratio and wavelength coverage to previously published data, so a dditional spectral features can be measured. Photometry obtained on the sam e nights is used to confirm the ephemeris and for comparison with light cur ves from previous years. Analysis of the color variation through the orbita l cycle has been carried out using archival MACHO data.(2) When a barely re solved red held star is accounted for, there is no (V-R) color variation, e ven through eclipse. There have been substantial changes in the depth of mi nimum light since 1988; it has decreased more than 0.5 mag in the last seve ral years. The spectral features and radial velocities are also found to va ry not only through the 0.44 day orbit but also over timescales of a year o r more. Possible interpretations of these long-term changes are discussed. The 1996 spectra contain phase-modulated Balmer absorption lines not previo usly seen, apparently arising in gas flowing from the region of the compact star. The changes in emission-line strengths with orbital phase indicate t hat there are azimuthal variations in the accretion disk structures. Radial velocities of several lines give different amplitudes and phasing, making determination of the stellar masses difficult. All solutions for the stella r masses indicate that the companion star is considerably less massive than the degenerate star. The Balmer absorption-line velocities correspond to m asses of similar to 1.4 M. for the degenerate star and similar to 0.4 M. fo r the mass donor. However, the strong He II emission lines indicate a much more massive accreting star, with M-x > 4 M..