4 Conclusions

We have presented some new results of CCD photometry for 7 BL Lac objects. All of them exhibited microvariations during our observation. The XBL, H 0323+022, exhibited brightness oscillations on 15 December 1996 and 05 January 1997. The largest amplitude is about 0.63 mag. Another XBL, H $0548\, -\, 322$, exhibited smaller oscillation on 01 January 1997. Although in our observation the XBL H $2154\, -\, 304$ presented no brightness oscillation, there are reports about oscillation of about one day in this source (Smith et al. 1992; Courvoisier et al. 1995). During our observation, the four RBLs, PKS 0735+178, PKS 0754+101, OJ 287 and BL Lac did not show this kind of quick brightness oscillations or one-after-another flickers and then do not microvary very frequently. Carini et al. (1992b) have investigated microvariability in OJ 287 and BL Lac extensively. Their observation also recorded no oscillation of about one day or less, and microvariations in these two source do not appear very frequently. Dultzin-Hacyan, Takalo & Benitez et al. (1997) have observed OJ 287 during a flare and found six microvariations on six nights out of ten nights. The largest amplitude is about 0.3 mag and the microvariations do not appear very frequently. Microvariations in other RBLs, e.g. 0954+658 (Wagner et al. 1993), AP Librae (Carini et al. 1990a), OQ 530 (Carini et al. 1990b), 3C 66A (Takalo & Sillanp$\rm\ddot{a}\ddot{a}$ et al. 1996), AO 0235+164 (Rabbette et al. 1996), also do not appear very frequently. The only RBL that shows daily brightness oscillations is S5 0716+714 (Wagner et al. 1994). It thus seems that RBLs do not show microvariability more frequently than XBLs. If the relativistic jets in RBLs truly orient closer to the line of sight than the relativistic jets in XBLs do, then RBLs should have higher Doppler factors than XBLs, and those microvariations that are undetectable in XBLs should become detectable in RBLs. That is to say, RBLs should microvary more frequently than XBLs. However, variability behaviour in BL Lac objects is very complex, and even in a flare the microvariability behaviour during the onset of the flare and during the decline part of the flare is different (Dultzin-Hacyan et al. 1997). Therefore, it is necessary to carry out more photometric observations for BL Lac objects to further investigate whether RBLs do not microvary more frequently than XBLs.

Acknowledgements

We are grateful to National Science Foundation of China and ULOAC for their support of this work.