RX J0537.7-7034: The shortest-period supersoft X-ray source

We present new photometric and spectroscopic observations of the transient supersoft X-ray source RX J0537.7-7034 and find a periodicity of approximat ely 3.5 hrs. This establishes RX J0537.7-7034 as the supersoft X-ray source with the shortest orbital period. We furthermore derive an inclination of the binary system of 45 degrees less than or similar to i less than or simi lar to 70 degrees, and the masses Of the two binary components: M-accretor = 0.6 +/- 0.2 M-circle dot, M-donor = 0.35 +/- 0.02 M-circle dot. This impl ies that the standard scenario of supersoft X-ray sources, in which the don or is thought to be more massive than the accreting white dwarf to ensure h igh mass transfer rates on a thermal timescale (van den Heuvel et al. 1992) , is not applicable for this system. We discuss alternative interpretations of this source as a former nova in which the thermonuclear flashes have be come mild and most accreted mass is retained by the white dwarf (so-called SMC 13 type systems), or as a self-sustained wind-driven system.