  We present 10 years of R-band monitoring data of 31 northern blazars which
were either detected at very high energy (VHE) gamma rays or listed as
potential VHE gamma-ray emitters. The data comprise 11820 photometric data
points in the R-band obtained in 2002-2012. We analyze the light curves by
determining their power spectral density (PSD) slopes assuming a power-law
dependence with a single slope $\beta$ and a Gaussian probability density
function (PDF). We use the multiple fragments variance function (MFVF) combined
with a forward-casting approach and likelihood analysis to determine the slopes
and perform extensive simulations to estimate the uncertainties of the derived
slopes. We also look for periodic variations via Fourier analysis and quantify
the false alarm probability through a large number of simulations. Comparing
the obtained PSD slopes to values in the literature, we find the slopes in the
radio band to be steeper than those in the optical and gamma rays. Our
periodicity search yielded one target, Mrk 421, with a significant (p<5%)
period. Finding one significant period among 31 targets is consistent with the
expected false alarm rate, but the period found in Mrk~421 is very strong and
deserves further consideration}.
