Long and short term variability of seven blazars in six
  near-infrared/optical bands
  Context. We present the light curves of six BL Lac objects, PKS 0537-441, PKS
0735+17, OJ 287, PKS 2005-489, PKS 2155-304, W Comae, and of the flat spectrum
radio quasar PKS 1510-089, as a part of a photometric monitoring program in the
near-infrared/optical bands started in 2004. All sources are Fermi blazars.
Aims. Our purpose is to investigate flux and spectral variability on short and
long time scales. Systematic monitoring, independent of the activity of the
source, guarantees large sample size statistics, and allows an unbiased view of
different activity states on weekly or daily time scales for the whole
timeframe and on nightly timescales for some epochs. Methods. Data were
obtained with the REM telescope located at the ESO premises of La Silla
(Chile). Light curves were gathered in the optical/near-infrared VRIJHK bands
from April 2005 to June 2012. Results. Variability >= 3 mag is observed in PKS
0537-441, PKS 1510-089 and PKS 2155-304, the largest ranges spanned in the
near-infrared. The color intensity plots show rather different morphologies.
The spectral energy distributions in general are well fitted by a power law,
with some deviations that are more apparent in low states. Some variability
episodes during a night interval are well documented for PKS 0537-441 and PKS
2155-304. For the latter source the variability time scale implies a large
relativistic beaming factor.
