1 Introduction

V Sge is a close binary ($P_{\rm orb}=0.514$ d) with peculiar spectroscopic and photometric characteristics [11, (Herbig et al. 1965,] hereafter HPSP). The large amount of circumstellar matter around V Sge manifests itself by very strong complicated emission lines and precludes resolution of any photospheric absorption line (HPSP, [33, Robertson et al. 1997;] [28, Patterson et al. 1998;] [6, Gies et al. 1998).] Also the orbital light curve and its variations are very likely affected by this matter. Due to these circumstances any direct analysis of the components of V Sge (and even a pure determination of the system configuration) is very difficult. There are still two basically different models for V Sge: (a) Algol-type binary, consisting of two non-degenerate stars, the lobe filling primary and detached secondary (HPSP), modified by [48, Wood & Lockley (1997)] who argued that V Sge is a colliding-wind binary; (b) white-dwarf (WD) primary accreting mass through a disk [46, (Williams et al. 1986;] [28, Patterson et al. 1998;] [39, Steiner & Diaz 1998);] V Sge would be a candidate for the super-soft X-ray source (see [44, van den Heuvel et al. 1992] for definition).

The gradual decrease of the orbital period of V Sge may be due to the mass transfer [38, (Smak 1995;] [41, Simon 1996a;] [22, Mader & Shafter 1997;] [28, Patterson et al. 1998).] The rate of its change is small, of the order of $\Delta{P}/{P}\approx 10^{-10}$ day$^{\rm -1}$. Here it must be admitted that the mass transfer rate quoted by [41, Simon (1996a)] is in error. The correct value is near that given by [38, Smak (1995)] ($\dot {m}\approx 10^{-7}\;M_{\odot}$ yr^-1).

V Sge has been detected in a broad spectral range, from X-ray [3, (Eracleous et al. 1991;] [13, Hoard et al. 1996)] and UV [18, (Koch et al. 1986)] to radio [21, (Lockley et al. 1997).] The latter observations imply a very high mass transfer or outflow rate of the order of $10^{-6}\;M_{\odot}$ yr^-1. Very strong photometric activity has attracted most attention (HPSP, [42, Simon 1996b] - hereafter Paper I, [33, Robertson et al. 1997).] In this paper, a preliminary version of which was presented by [43, Simon & Mattei (1998),] we will further analyse mainly the overall characteristics and long-term evolution of this activity. We will also try to utilize the character of the long-term changes for putting V Sge into broader context of the interacting binaries.