  We present an analysis of recent XMM-Newton and HST archive data of the
ultraluminous X-ray source NGC 1313 X-2. Quasi-simultaneous observations taken
with XMM-Newton, HST and VLT allow us to study both the X-ray light curve and
its correlation with the optical emission of the two proposed ULX counterparts.
At the end of December 2003 the source experienced a short, but intense flare,
reaching a maximum luminosity of ~10^40 erg/s. At the same time, the optical
flux of both the suggested counterparts did not show pronounced variations
(<30%). Assuming that the ULX emission is isotropic and taking X-ray
reprocessing into account, the optical data for one of the proposed
counterparts are consistent with it being an early type, main sequence star of
about 10-18 Msun losing matter through Roche-lobe overflow onto a ~120 Msun
black hole at an orbital separation corresponding to a period of about 2 days.
