  In this paper we present results of a short-term optical monitoring of 13
blazars. The objects were monitored mostly in the R-band for a total of ~ 160
hours between 2006 and 2011. We study the nature of the short-term variations
and show that most of them could be described as slow, smooth, and (almost)
linear changes of up to ~ 0.1 mag/hour, but many objects show no short-term
variations at all. In fact, we found only ~ 2 per cent chance to observe
variability of more than 0.1 mag/hour for the sample we observed. Hints for
quasi-periodic oscillations at very low amplitude levels are also found for
some objects. We briefly discuss some of the possible mechanisms to generate
the intra-night variability and the quasi-periodic oscillations.
