Variability studies of seyfert galaxies-I. Broad-band optical photometry

We present the results of a four-year campaign to monitor the variations of 35 southern Seyfert galaxies, using UBV(RI)C multi-aperture photometry. Most of the galaxies are found to be variable. The light curves are displayed and some of their more interesting details are discussed. The relationships between the fluxes through different filters are well described by straight lines, which suggests that the spectral shape of the variable part of the optical continuum stays constant. The values of the slopes are used to derive the shapes of the continua, the extinctions and the luminosities of the Seyfert nuclei. It is shown that the nuclear flux in the optical is largely of thermal origin. The significance of the results to current models of Seyfert nuclei is discussed.