The BL Lacertae objects (BL Lacs) belong to the class of blazars, which are the active galactic nuclei showing the most extreme features: intense and variable emission and large and variable polarization. According to the current interpretation, their radiation is produced in a relativistic jet and is beamed towards us. However, many important questions about the mechanisms that are responsible for the emission at the various wavelengths are still to be answered and a big observational effort is required to shed light on this matter. In recent years it has become clear that useful information can be derived by the simultaneous observation of these objects in different bands, in order to reveal possible correlations and to discriminate among the theoretical models. The launch of satellites for astronomical studies has made this kind of study possible.
Since fall 1994 the Torino blazar monitoring group is performing optical observations of blazars, in particular during satellite pointings (see e.g. Villata et al. 1997; Raiteri et al. 1998; Ghisellini et al. 1997). In this paper we present BVR data for eight objects that are part of the X-ray selected BL Lacs core program of the Satellite per Astronomia X "Beppo" (BeppoSAX). Observations were done during or close to the satellite pointings. The X-ray data are reported in Wolter et al. (1998), where the spectral energy distributions of all the eight BL Lacs are also shown.
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