The optical (BVRI) monitoring data covering 5 years for 12 x-ray-selected BL Lac objects are presented in this paper. The main results are: (1) IE 0317+178 has shown a variability of 0.53 mag in R band on a time scale of about 1.93 days. (2) A variability of 0.58 mag within 5 min was recorded in the V band for H 0548-322. Our results would be valuable to understand the relation between XBLs and RBLs. (C) 1996 American Astronomical Society.
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