HUBBLE-SPACE-TELESCOPE ULTRAVIOLET SPECTROSCOPY OF THE SUPERSOFT X-RAY BINARIES CAL83 AND RXJ0513.9-6951

We present Hubble Space Telescope ultraviolet observations with the Go ddard High Resolution Spectrograph of the supersoft X-ray binaries CAL 83 and RXJ0513.9-6951 in the Large Magellanic Cloud. Both sources sho w a remarkably similar spectrum with an almost flat continuum and weak NV and OV emission lines. The neutral hydrogen column densities deriv ed from the broad Ly alpha profile are N-HI = (6.5 +/- 1.0) x 10(20) c m(-2) for CAL 83 and N-HI = (5.5 +/- 1.0) x 10(20) cm(-2) for RXJ0513. 9-6951. These column densities are very similar to the galactic foregr ound column density, indicating that the two sources are located on th e near side of the LMC. With this independent estimate of N-HI, we fin d that the X-ray source in CAL 83 has a luminosity of (0.7 - 2) x 10(3 7) ergs s(-1). The inferred radius for the supersoft source in CAL 83 is consistent with the radius of a white dwarf which has not significa ntly expanded due to the shell burning. For RX J0513.9-6951, we find a luminosity of (2.5 - 9) x 10(37) ergs s(-1). The long-term ultraviole t light curve of CAL 83 shows a rare bright state during which the ult raviolet flux has doubled.