The optical variability of seven BL Lacertae objects

UBV photographic magnitudes and colour indices have been obtained during a continuous 6 year monitoring program. The main results show that: (1) Mark 421 has a very short characteristic time scale of about 2.5 hours for the optical variability with an amplitude of ΔB = 0.48 mag. (2) ON +325 has a characteristic time scale of about 3 days for the optical variability with an amplitude of ΔB = 0.38 mag. (3) OJ 287 has shown a variability on a short time scale of about 8 days with an amplitude of ΔB = 0.37 mag. (4) From 1980 to the end of 1982, OJ 287 has been stable and fainter, the faintest magnitude being B = 15.61 mag. However, an outburst of B = 14.25 mag was recorded in March 1983. (5) The authors proved that the smallest characteristic time scale of the optical variability for a galaxy is 10 minutes.