2 Observations and data reduction

The observations were done with the $1.05 \, \rm m$ REOSC telescope of the Torino Astronomical Observatory, equipped with a $1242 \times 1152$ pixel CCD camera (EEV) and standard Johnson's BV and Cousins' R filters. Data were taken during 14 photometric nights, from February 1995 to May 1997.

Frames were reduced by the Robin procedure locally developed, which includes bias subtraction, flat fielding, and circular gaussian fit after background subtraction.

Calibration was obtained through observations of Landolt's fields (Landolt 1992) and other standard stars during each night. The Calib procedure was written to transform instrumental magnitudes into standard ones; the relevant equations are

$$V=v_0+\zeta_v+\epsilon(V-R),$$ (1)

$$V-R=\psi\left(v_0-r_0\right)+\zeta_{vr},$$ (2)

$$B-V=\mu\left(b_0-v_0\right)+\zeta_{bv},$$ (3)

where $\zeta$ are the zero-point constants; $\epsilon$, $\psi$, $\mu$ are the transformation coefficients; and v₀, r₀, b₀ are the instrumental magnitudes corrected for atmospheric extinction:

v₀=v-k'_vX ,

$$r_0=r-\left[k'_r+k''_r(v-r)\right]X ,$$ (5)

$$b_0=b-\left[k'_b+k''_b(b-v)\right]X ,$$ (6)

where v, r, b are the instrumental magnitudes, k' and k'' represent the principal and second-order extinction coefficients, and X is the air mass.

In one night V observations were lacking so that we had to base the transformation to the standard system on the B and R bands only; the resulting magnitudes were in agreement with those obtained in the other nights.