We present VRc Ic Johnson-Cousins magnitudes of field comparison stars for twelve BL Lacertae objects belonging to the largest sample of blazars which is monitored at the Perugia University Observatory. The sequences reported here can be used in the long–term CCD photometric monitoring of BL Lac objects aimed to understand the characteristics of their variability. Finding charts for all of these comparison sequences are also included in this paper.
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Fiorucci et al. (1996) studied this question.