Our data suggest that at least for DX And, AL Com, V660 Her and SY Cnc (Spogli et al. 1993) steady-state accretion disk models do not provide a good representation of the optical continuum. This is a clear evidence about our poor knowledge of the true radiative transfer solution in accretion disk, and more efforts must to be made to obtain multi-wavelength observations of DNs.
These considerations show how important it is to study the dwarf novae during all the phases of the outburst, integrating photometric multi-band observations with the aim of studying both the behaviour of the light curve and the spectral distribution of the emitted flux. This work requires a continuous monitoring of a sample of sources and, therefore, it cannot be done with the great astronomical telescopes, because the limited availability of time that is given to every type of study. On the other hand, having to observe quite bright objects, we can use small telescopes, which are available and well suitable to carry out programs of long-term CCD photometry of dwarf novae. In this context the observations obtained at the Astronomical Observatory of Perugia can be considered an example of how a small telescope can be used to produce scientific data.
Acknowledgements
We would like to thank the anonymous referee for the careful reading of this manuscript and the important comments and suggestions which led to a more complete and useful paper. We are also indebted to Paolo Maffei without whose encouragement and support this work would have never been possible.
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