1 Introduction

BL Lacertae objects (BL Lacs) are radio-loud active galactic nuclei (AGNs) characterized by a very variable emission at all wavelengths. They belong, together with a subset of radio-loud quasars, to the class of blazars (Urry & Padovani 1995).

Optical monitoring represents the possibility of following their optical emission behaviour with the aim of understanding the mechanisms of such variability. In the last years many efforts have been spent to organize international monitoring campaigns in order to improve the monitoring efficiency. Moreover, simultaneous observations at different wavelengths are demanded, since the discovery of possible correlations among the emissions in different bands can be an important key to clarify the radiation processes. The operation of several satellites for astronomical researches, such as the Compton Gamma Ray Observatory (CGRO) and the Satellite per Astronomia X "Beppo'' (BeppoSAX), has recently brought much excitation on this kind of research.

The Torino monitoring group was formed in fall 1994 with the aim of observing $\gamma$-ray loud blazars and of providing the optical counterpart to the CGRO pointings. From then on, the object list has been extended to include other interesting sources, many of which belonging to the BeppoSAX programs. For further information on our monitoring activity and collaborations we refer to Villata et al. (1997); Massaro et al. (1996); Ghisellini et al. (1997); Raiteri et al. (1998a); Tosti et al. (1998).

In this paper we present BVR calibration of 45 stars and VR calibration of 11 stars in the fields of 10 BL Lacs that are part of our observational list. Photometric sequences for 9 quasars will be published in a forthcoming paper (Raiteri et al. 1998b).