THE LARGE-MAGELLANIC-CLOUD SUPERSOFT X-RAY BINARY RX-JO513.9-6951

A detailed analysis of simultaneous photometric and spectroscopic obse rvations of the optical counterpart of the LMC ''supersoft'' X-ray sou rce RX J0513.9-6951 (identified with HV 5682) is presented. The spectr um is dominated by He II emission lines and H + He II blends; no He I is observed, but several higher ionization emission features, especial ly O VI (3811, 3834, and 5290 Angstrom), are prominent. Radial velocit y measurements suggest a binary period of 0.76 days. If the small velo city amplitude, K similar to 11 km s(-1) is interpreted as orbital mot ion, this implies that the binary system contains a somewhat evolved s tar plus a relatively massive compact object, viewed nearly pole-on. N o orbital photometric variations were found, although irregular bright ness changes of similar to 0.3 mag occurred. Unusual emission lines ar e observed that cannot be identified except as high-velocity (4000 km s(-1)) bipolar outflows or jets. These outflows are seen in H and He I I at the same positive and negative velocities. They were relatively s table for periods of about 5 days, but their velocities appear to have been about 250 km s(-1) smaller in 1992 than in 1993 or 1994.