1 Introduction

Cataclysmic variable stars (CVs) are binary systems with a white dwarf primary and a late-type main sequence secondary star in orbit around each other with periods of only a few hours. The secondary star fills its Roche lobe and the mass it loses through the inner Lagrangian point forms an accretion disk around the white dwarf. In CVs accretion luminosity is usually the dominant contribution to the total light of the system.

Dwarf novae (DNs) are a sub-class of cataclysmic variables characterized by the presence of sudden increases of brightness (outbursts) in the optical light curve. The outbursts have amplitudes in the range 2-5 mag and they occur at irregular intervals of time, typically ranging from about ten days to some months. In the case of dwarf novae, the disk instability model seems to account for most of the properties observed during a normal outburst (see, e.g., Meyer & Meyer-Hofmeister 1994).

Sometimes some dwarf novae show "superoutbursts'' in the light curve, which are characterized by a larger amplitude and a longer duration than normal outbursts. Superoutbursts occur at regular, but not strictly periodic intervals of time (see, e.g., Osaki 1996). In order to explain these superoutbursts, three different models have been proposed: the enhanced mass transfer model, the thermal limit cycle model, and the thermal-tidal instability model (for a complete review, see Osaki 1996, and references therein).

So far, most of the optical observations of dwarf novae were carried out photographically or have been supplied by amateur astronomers through visual estimations, and multi-wavelength campaigns have been carried out only for a few of the brightest DNs (see, e.g., Echevarria et al. 1996). Indeed, multi-band monitoring is of special interest in order to extend the work done by amateurs, to study the spectral behavior of the optical continuum, and to explore the physics of accretion disks. For this reason we included a small sample of dwarf novae as a secondary target in our automatic monitoring program of variable sources.

  

Table 1: Coordinates and historical informations of the five selected objects$^\star$

Notes: a) UG=U Gem variable, UGSS=U Gem variable (SS Cyg subtype), DQ=DQ Her variable; b) maximum apparent magnitude; c) minimum apparent magnitude; d) magnitude systems: v=visual, p=photographic.
$\star$ extracted from the catalogue of Downes & Shara (1993).

We here present the photometric observations of five DNs (see Table 1) which showed a high level of activity during the period October 1994 - July 1995.