SIMBAD references

We present the results of our monitoring the flux variability of 12 BL Lac objects, which have variabilities on time-scales ranging from hours to months. Individual sources are discussed in detail. Three of them, OY091, H0548-322 and 0048-097, show significant rapid variation (hours). Two of them, 3C 66A and Mrk 501, exhibit significant variability on time-scales of months. We find that 3C 66A has a variability period of 63±5 d, supporting the 65-d period obtained by Lainela et al. The periodicity seems to be weak according to the Jurkevich V^2_m test. We also find a correlation between B-V colour index and B magnitude for Mrk 421. We discuss possible physical mechanisms on the basis of these observational phenomena.