The nature of the intra-night optical variability in blazars
  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. 
R. Bachev (1), E. Semkov (1), A. Strigachev (1), Alok C. Gupta (2),   Haritma Gaur (2,3), B. Mihov (1), S. Boeva (1), L. Sla...
astro-ph.HE astro-ph.CO