Photometric monitoring of three BL Lacertae objects in 1993-1998

The results of optical photometric (BV RI) monitoring of three BL Lac objec ts over a time interval of about four years are presented. The sources are three classical radio-selected BL Lac objects, BL Lac, OJ 287 and PKS 0735178. During our observation OJ 287 was in the stage of a large periodic out burst which consisted of at least two peaks. Almost all the observations ob tained over consecutive nights detected intranight variations. In 1995 and 1996 BL Lac kept in faint states, with fewer and smaller rapid flares and f luctuations. On the contrary, in late 1997 BL Lac was at the stage of a lar ge outburst, accompanied with much more large amplitude rapid flares and fl uctuations. PKS 0735+178 was almost at its faint end from 1994 to early 199 8. Over this time interval, the intraday variations and microvariations in PKS 0735+178 were rare and the amplitude was very small, except a rapid dar kening of similar to 0.4 mag on 24 January 1995. Previous work by Webb et a l. (1988); Wagner et al. (1996); Plan et al. (1997) also showed the same be haviour of variability as BL Lac and PKS 0735+178 in BL Lac, S5 0716+714, P KS 2155 - 304, respectively. We propose that the motion of orientation of t he relativistic jet in a BL Lac object be responsible for these variability behaviours.