We have obtained spectroscopy with the Far Ultraviolet Spectroscopic Explorer (FUSE) of the supersoft X-ray binary RX J0513.9−6951 over a complete binary orbital cycle. The spectra show a hot continuum with extremely broad O VI emission and weak Lyman absorptions. He II emission is weak and narrow, while N III and C III emissions are undetected, although lines from these ions are prominent at optical wavelengths. The broad O VI emission and Lyman absorption show radial velocity curves that are approximately antiphased and have semiamplitudes of ∼ 117 ± 40 and 54 ± 10 km s −1 , respectively. Narrow emissions from He II and O VI show small velocity variations with phasing different from the broad O VI, but consistent with the optical line peaks. We also measure considerable changes in the FUV continuum and O VI emission line flux. We discuss the possible causes of the measured variations and a tentative binary interpretation.

Far-ultraviolet spectra of LMC X-3 were taken covering photometric phases 0.47-0.74 in the 1.7 day orbital period of the black hole binary (phase zero being superior conjunction of the X-ray source). The continuum is faint and flat but appears to vary significantly during the observations. Concurrent RXTE/ASM observations show the system was in its most luminous X-ray state during the Far Ultraviolet Spectroscopic Explorer (FUSE) observations. The far-UV spectrum contains strong terrestrial airglow emission lines, while the only stellar lines clearly present are emissions from the O vi resonance doublet. Their flux does not change significantly during the FUSE observations. These lines are modeled as two asymmetric profiles, including the local ISM absorptions due to C ii and possibly O vi. Velocity variations of O vi emission are consistent with the orbital velocity of the black hole and provide a new constraint on its mass.

A new ephemeris has been determined for the supersoft X-ray binary CAL 83 using MACHO Project photometry. With an improved orbital period of 1.047568 days, it is now possible to phase together photometric and spectroscopic data obtained over the past two decades with new far-ultraviolet spectra taken with the Far Ultraviolet Spectroscopic Explorer. We discuss the properties of the orbital and long-term optical light curves, as well as the colors, of CAL 83. In the far-ultraviolet, the only well-detected stellar feature is emission from the O vi resonance doublet. The radial velocity of this emission appears to differ from that of He ii in the optical region, although we only have partial phase coverage for the O vi line. The FUSE continuum variations are similar to the optical light curve in phase and amplitude.

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