The timescales of the optical variability of blazars. III - OJ 287 and BL Lacertae

The BL Lacertae objects OJ 287 and BL Lac have been photometrically monitored in an effort to study the nature of optical variations which may occur on timescales ranging from years to less than a day. The results of ten years of photometric monitoring of these two objects show variations which are consistent with those reported by other authors. No strong dependence of color with source brightness was detected, although both sources exhibited a weak tendency to be bluer when brighter. Microvariability was observed for both objects; variations as large as 0.1 mag/hr were observed for BL Lac and changes as large as 0.08 mag/hr were observed for OJ 287. No evidence for a periodicity was found in the observed variations of either object.