1999A&A...343..183G

We have observed V 751Cyg with the ROSAT HRI in a target-of-opportunity mode during its recent optical low state and clearly detect it at a count rate of 0.015cts/s. The X-ray intensity is a factor of 7-19 (depending on the exact X-ray spectral shape) higher than the upper limit obtained with the ROSAT PSPC during the optical high state, thus suggesting an anti-correlation of X-ray and optical intensity. Spectral investigations suggest a very soft X-ray spectrum. We investigate archival IUE data of V751 Cyg and derive a distance of V751 Cyg of d≃500pc based on the extinction estimate of E(B-V)=0.25±0.05. This implies that the X-ray emission is very luminous, on the order of 10³⁴-10³⁶erg/s. We have obtained quasi-simultaneous optical photometry and spectroscopy. The spectrum during the optical low-state is characterized by a very blue continuum and the presence of strong emission lines of the Balmer series and HeI. Also, HeII 4686Å is clearly detected. We establish that V751 Cyg is a transient supersoft X-ray source and speculate that other VY Scl stars may also be of similar type.