2 Observations and data reduction

All data were taken with the $1.05 \, \rm m$ Cassegrain reflector of the Torino Astronomical Observatory. The instrumentation comprises a $1242 \times 1152$ pixel CCD camera (EEV) and standard Johnson's BV and Cousins' R filters. The observations were done during 14 photometric nights, from February 1995 to May 1997. Flat field and bias frames were taken each night for the image correction; no dark current correction is needed. Several Landolt's fields (Landolt 1992) and other standard stars were observed during each night.

Frame reduction was performed with the Robin procedure locally developed, including bias subtraction, flat fielding, and circular gaussian fit after background subtraction.

Transformation of the instrumental magnitudes into standard ones was obtained with the Calib procedure (see Paper I for a description of the transformation equations).