The Large Magellanic Cloud supersoft X-ray binary RX JO513.9-6951

A detailed analysis of simultaneous photometric and spectroscopic observations of the optical counterpart of the LMC ''supersoft'' X-ray source RX J0513.9-6951 (identified with HV 5682) is presented. The spectrum is dominated by He II emission lines and H + He II blends; no He I is observed, but several higher ionization emission features, especially O VI (3811, 3834, and 5290 Angstrom), are prominent. Radial velocity measurements suggest a binary period of 0.76 days. If the small velocity amplitude, K similar to 11 km s(-1) is interpreted as orbital motion, this implies that the binary system contains a somewhat evolved star plus a relatively massive compact object, viewed nearly pole-on. No orbital photometric variations were found, although irregular brightness changes of similar to 0.3 mag occurred. Unusual emission lines are 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 II at the same positive and negative velocities. They were relatively stable 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.