5 Concluding remarks and implications

We argue that the CV-like model, comprising the mass accreting WD primary, and variable mass transfer offer a viable explanation for the photometric activity of V Sge. Our analysis therefore brings an additional support to the model by [46, Williams et al. (1986)] and [28, Patterson et al. (1998).]

In this context, some previous findings should be briefly rediscussed. Absence of rotational disturbance during the primary eclipse in the spectra of [33, Robertson et al. (1997)] is not necessarily in contradiction with the model of the accreting WD primary. We can offer explanations in several ways: their data were obtained during HS (segment S7); the enhanced mass transfer in HS can lead to saturation of the amount of the circumstellar matter in the lobe of the primary, even with a possible formation of an extended envelope - the line profiles suggest a large amount of uneclipsed matter. Moreover, the emission structures in V Sge are highly variable, often persist just for several orbital cycles [33, (Robertson et al. 1997;] [28, Patterson et al. 1998;] [6, Gies et al. 1998)] and bear many links with the super-soft X-ray binary CAL 87 [16, (Hutchings et al. 1998).] The rotational disturbance is not visible even in some firmly established eclipsing CVs, lacking strong magnetic fields (e.g. [47, Williams 1989;] [45, Warner 1995).] Its absence therefore is not a conclusive argument against disk.

Also the orbital light curve of V Sge, mainly near LS, displays striking similarities to those of the eclipsing super-soft X-ray sources like CAL 87 and J0019.8 + 2156. The modified disk around WD, expected for a high $\dot m$, is able to reproduce the main features [27, (Meyer-Hofmeister et al. 1997;] [34, Schandl et al. 1997).] It thus offers an alternative to the two-star model by [22, Mader & Shafter (1997).]

We also may constitute a group of luminous CV-like systems with relatively long orbital periods ($P_{\rm orb}\gt 10$ hours), which display large brightness variations on the time scale of days to weeks. It includes V Sge, RX J0513.9 - 6951 [1, (Alcock et al. 1996)] and RX J0019.9 + 2156 [2, (Bartolini et al. 1996;] [7, Greiner & Wenzel 1995).] Since their changes often have a form of HSs and LSs, they are most likely invoked by variable $\dot m$. It suggests that there exists another region of the mass transfer instability, in addition to the range of $3<P_{\rm orb}<4$ hours within which most VY Scl-type systems are observed (e.g. [45, Warner 1995).]

Acknowledgements

This research has made use of observations primarily from the AAVSO International database (Massachusetts, U.S.A), and also from the AFOEV database (operated at CDS, France). We thank the variable star observers worldwide whose decades of observations made this analysis possible. We thank Dr. R. Hudec for reading the manuscript and for valuable comments. V.S. thanks Dr. S. Stefl for a discussion.