Search for short variability time-scales of BL Lacertae objects.

CCD image photometry data for 12 BL Lacertae objects have been obtained during 17 months. Light curves and photometric data are presented and discussed. The results suggest that a large amplitude variation ({DELTA}m >= 0.45 mag) on a short time scale (from a few hundred seconds to several hours) is a relatively rare phenomenon. Five of 12 BL Lac objects are variable on these time scales. Evidences for relativistic beaming in 5 BL Lac objects are discussed.