THE NATURE OF THE SUPERSOFT X-RAY SOURCE RX J0513-69

We present spectroscopy and photometry of the LMC supersoft binary sys tem RX J0513.9-6951. We derive a refined spectroscopic period of P = 0 .761 +/- 0.004 day, which is consistent with the value obtained from l ong-term photometric monitoring (P = 0.76278 +/- 0.00005 day). We see bipolar outflow components of He II and H beta, with velocities of sim ilar to 3800 km s(-1), strongly suggesting that the compact object is a white dwarf. Using all the available optical and X-ray data, we cons truct a theoretical model to explain the principal features of the unu sual variability of this source. In particular, we note that X-ray out bursts have only been seen at times of optical minima. From this, we c onclude that the most likely cause of the X-ray outbursts is a photosp heric contraction during a nuclear shell burning phase, rather than a thermonuclear flash or shocked emission. The system probably comprises a relatively massive white dwarf accreting at a high rate (similar to 10(-6) M(.) yr(-1)) from an evolved donor star and is observed close to pole-on.