CCD photometry of five X-ray selected BL Lacertae objects

We present the results of the monitoring of a sample of 5 HBLs in the B, V and R bands during 2003–2008. All sources in our monitoring project show significant rapid optical variations, except 1ES 1959+650, however; for 1ES 1959+650, the gamma-ray variability on a timescale of ∼7 hours has been observed by Holder et al. (Astrophys. J. 583:L9, 2003 ). Using these variability timescales, we estimate the black hole masses for each source with the Kerr black hole theory.