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Published online by Cambridge University Press: 19 July 2016
A radio-selected (1 Jy, Stickel et al., 1991) and a x-ray selected (EXOSAT, Giommi et al., 1991) sample of BL Lac objects was monitored by performing relative CCD photometry in order to examine the duty cycle, the dominant time-scales and the typical amplitudes of the variability. The samples consist of 34 and 11 objects, respectively.