2005A&A...437...87D

We present results of UBVRI photometry of Mrk 335 in 1995-2004. This object has a large amplitude of variability, reaching about 1.1, 0.9, 0.7mag in the U,B,V and 0.3mag in the Rc,Ic bands, respectively. This photomerical behaviour as well as the spectral energy distribution in the optical part of the spectrum is typical of Seyfert 1 galaxies. The structure function (SF) analysis of Mrk 335 has shown that the variability of Mrk 335 can be caused by stochastic superposition of independent flares in a wide interval of durations, the longest of which has a duration of about several hundred days. The slope of the power-law portion of the SF b∼0.6-0.7 for the time scale from several days to several hundreds of days. It is close to the estimates for disk instability models. The estimate of the extension of the instability zone in an accretion disk (about 300R_S) is also evidence that the instability in an accretion disk is able to explain the long-term optical variability. A cross-correlation function (CCF) shows some evidence that the delay of fluxes in the V,R,I bands relative to the U band can be of some days, and there is a tendency for lags to be systematically higher for the R, I bands than for B, V.