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Astron. Astrophys. 339, L21-L24 (1998)

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1. Introduction

V Sge is a blue star with a mean brightness around 11 mag which has been shown to vary between 9.6-14.7 mag since its discovery in 1902. It shows wide eclipses at a period of [FORMULA], a small secondary eclipse, and complex emission line behaviour (Herbig et al. 1965). Extinction estimates vary between [FORMULA] (Herbig et al. 1965) and [FORMULA] (Verbunt 1987) implying a distance of 0.7-2.7 kpc.

Supersoft X-ray binaries (SSB; see Greiner 1996 and references therein; van Teeseling 1998) were established as a new class of accreting binaries during the early 90ies with ROSAT (Trümper et al. 1991; Greiner et al. 1991) and are thought to contain white dwarfs accreting mass at rates sufficiently high to allow stable nuclear surface burning of the accreted matter (van den Heuvel et al. 1992). SSB have luminosities of [FORMULA] 1036-1038 ergs s-1, but their characteristic temperatures of 20-40 eV imply strong attenuation by the interstellar medium. Thus, most of the known SSB are located in external galaxies (e.g. Greiner 1996) making detailed optical observations difficult. It is therefore of great interest to identify galactic SSB.

It has recently been suggested (Steiner & Diaz 1998; Patterson et al. 1998) that V Sge has spectroscopic and photometric properties which are very similar to those of SSB. This suggestion is based on characteristics which are typical for SSB, but are rare or even absent among canonical cataclysmic variables: (1) the presence of both OVI and NV emission lines, (2) a He II [FORMULA]/H[FORMULA] emission line ratio [FORMULA], (3) rather high absolute magnitudes and very blue colours, and (4) orbital lightcurves which are characterized by a wide and deep eclipse.

The suggestion of the similarity of V Sge to SSB is almost entirely based on optical and ultraviolet data. In this paper, we investigate the archival ROSAT data of V Sge and discuss them in the context of the long-term optical behaviour of V Sge. Hoard et al. (1996) reported the detection of V Sge as a soft X-ray source in the Nov. 1992 ROSAT observation, but did not perform a spectral fit. Verbunt et al. (1997) already reported the non-detection of V Sge during the ROSAT all-sky survey.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998