Optical low states of the supersoft X-ray source RX J0513.9-6951

We report the results of our optical multicolour monitoring campaign of the supersoft X-ray source RXJ0513.9-6951. During this campaign, RXJ0513.9-6951 went through an optical low-state, lasting similar to 40 days. The decrease of the optical flux, Delta V similar to 1 mag, and the accompanying reddening, Delta(B-V) similar to 0.1-0.2 mag and similar to(V-R) similar to 0.1 mag, can be quantitatively described by variations in the irradiation of the accretion disk by the hot central star. In this simple model, we consider a white dwarf with nuclear burning of accreted matter. Its photospheric radius shrinks in response to a temporarily slightly reduced mass-transfer rate. The same model can account for the temporarily increased soft X-ray flux observed twice with ROSAT and we expect that during the optical low-state the system turned on again as a soft X-ray source in the 0.1-2.4 keV energy band.