Transient supersoft X-ray emission from V 751 Cygni during the optical low-state

We have observed V 751 Cyg 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 0.015 cts s 1 . 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 suggest- ing an anti-correlation of X-ray and optical intensity. Spectral investigations suggest a very soft X-ray spectrum. We inves- tigate archival IUE data of V 751 Cyg and derive a distance of V 751 Cyg of d500 pc based on the extinction estimate of E(B V )=0.250.05. This implies that the X-ray emission is very luminous, on the order of 10 34 -10 36 erg s 1 . 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 4686 ˚ A is clearly detected. We establish that V 751 Cyg is a transient supersoft X-ray source and speculate that other VY Scl stars may also be of similar type.