All observations were performed with the 
telescope of the Torino
Astronomical Observatory. The instrument is equipped with a 
pixel CCD camera with scale 
per pixel. Data were reduced and
analyzed with MIDAS and the Robin procedure locally developed. More information
on the method can be found in Villata et al. (1996, 1997a).
Observations were done in the Cousins' R and Johnson's V and B
filters; weak sources were observed in the R band only. Exposure times
ranged from 
in the R band for the brightest objects (Mkn
421, 3C 273) to 
for the faintest ones.
Magnitude calibration has been obtained through comparison with photometric sequences in the same frame of the source. In the cases where a photometric sequence is missing, the source magnitude has been normalized to the magnitude of one reference star in the same frame. In this way a comparison with data of other observers can easily be performed by measuring the same star. Finding charts are available upon request to the authors to make the identification of these stars possible.