According to the current classification scheme for the active galactic nuclei, the class of blazars comprises BL Lacertae objects and a subset of quasars, all objects that are believed to emit their energy by a relativistic plasma moving towards us at a small angle to the line of sight. They are characterized by rapid variability, high and variable polarization, high brightness temperature, and superluminal motion of radio-emitting components (e.g. Urry & Padovani 1995).
Although several blazars have been monitored for many years, calibrated stars in the source field are still lacking for a number of them. This implies more work in order to obtain the blazar magnitude; moreover, using different reference stars may lead to different values of the blazar magnitude, thus making the comparison of data from different observers less meaningful than when a common photometric sequence is available. With this in mind, we calibrated field reference stars for a number of blazars that we are monitoring at the Torino Astronomical Observatory.
In this paper we present calibration of 46 stars in the fields of 9 quasars of our monitoring object list. Standard magnitudes for 56 stars in the fields of 10 BL Lacertae objects are published in Villata et al. (1998a; Paper I).
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