We present spectroscopy and photometry of the LMC supersoft binary system 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 long-term photometric monitoring (P = 0.76278±0.00005 day). We see bipolar outflow components of He II and Hβ, with velocities of ∼3800 km s-1, strongly suggesting that the compact object is a white dwarf. Using all the available optical and X-ray data, we construct a theoretical model to explain the principal features of the unusual variability of this source. In particular, we note that X-ray outbursts have only been seen at times of optical minima. From this, we conclude that the most likely cause of the X-ray outbursts is a photospheric 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 (∼10-6 Msun yr-1) from an evolved donor star and is observed close to pole-on.
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Southwell et al. (1996) studied this question.