The Eclipsing Supersoft X‐Ray Binary CAL 87

We present and discuss 25 spectra obtained in November 1996, covering allphases of the CAL 87 binary system. These spectra are superior both insignal-to-noise and wavelength coverage to previously published data so thatadditional spectral features can be measured. Photometry obtained on the samenights is used to confirm the ephemeris and to compare with light curves fromprevious years. Analysis of the color variation through the orbital cycle hasbeen carried out using archival MACHO data. When a barely resolved red fieldstar is accounted for, there is no (V-R)-color variation, even through eclipse.There have been substantial changes in the depth of minimum light since 1988;it has decreased more than 0.5 mag in the last several years. The spectralfeatures and radial velocities are also found to vary not only through the0.44-day orbit but also over timescales of a year or more. Possibleinterpretations of these long-term changes are discussed. The 1996 spectracontain phase-modulated Balmer absorption lines not previously seen, apparentlyarising in gas flowing from the region of the compact star. The changes inemission-line strengths with orbital phase indicate there are azimuthalvariations in the accretion disk structures. Radial velocities of several linesgive different amplitudes and phasing, making determination of the stellarmasses difficult. All solutions for the stellar masses indicate that thecompanion star is considerably less massive than the degenerate star. TheBalmer absorption-line velocities correspond to masses of ~1.4Msun for thedegenerate star and ~0.4Msun for the mass donor. However, the strong He IIemission lines indicate a much more massive accreting star, with Mx>4Msun.Comment: 18 pages including tables, plus10 figures. To appear in Ap