1 Introduction

Variability has always been used to probe into central engine and physical processes of active galactic nuclei (AGNs). In recent years, considerable progress has been made in the studies of variability in blazars. For variability on short timescales, observations especially those performed simultaneously in several wave bands for several blazars have obtained a lot of interesting results (e.g. Urry et al. 1993, 1997 and reference therein, Wagner & Witzel 1995 and reference therein, Villata et al. 1997). These studies have introduced new strict constraints for models on the structure of the center engine and physical processes of blazar. When explaining these new observational results, shocks-in-jet model seems to be more reasonable than other models (Wager & Witzel 1995). On the other hand, studies for long term light curves have shown variability periodicity in several sources, such as OJ 287 (Sillanp$\ddot{\rm a}\ddot{\rm a}$ et al. 1988; Kidger et al. 1992), 3C 345 (Schramm et al. 1993), 3C 273 (Babadzhanyants & Belokon 1992), ON 231 (Liu et al. 1995). During the long term monitoring from 1993 to 1996, OJ 287 was found to burst almost exactly at the predicted time, and a large amount of observational data with a very good time resolution and a long time span were obtained (Kidger et al. 1995; Sillanp$\ddot{\rm a}\ddot{\rm a}$ et al. 1996a, 1996b; Takalo et al. 1996). The periodicity of long timescale variability in blazars and irregularity of rapid variability indicates that the origin for long timescale variability is probably different from that for the short timescale variability.

In this paper, some new CCD photometric results since late 1993 for three BL Lac objects, BL Lac, OJ 287 and PKS 0735+178 are presented. BL Lac is the prototype of BL Lac objects. OJ 287 is a classical radio selected BL Lac object. It shows variability at all wavelength from radio to $\gamma$-rays. Its range of variability in the B band is as large as 5.5 mag, $18.0\geq B \geq 12.5$ mag (Takalo 1994). PKS 0735+178 is a highly variable BL Lac object. Historical light curves indicate that its range of variability in the B band is 13.90-17.72 mag (Bai & Xie et al. 1998). As a preliminary discussion of these data, we have compared the difference between variability behaviours in the high state and low state in BL Lac objects. We propose that the motion of orientation of the relativistic jet in a BL Lac object be responsible for these variability behaviours.