2 Observations

Our photometric observations were carried out with several telescopes: the 1.05 m astrometric telescope of the Torino Observatory, the 0.4 m Automatic Imaging Telescope (AIT) of the Perugia University Observatory, the 0.5 m telescope of the Astronomical Station of Vallinfreda, and the 1.0 m telescope at Tuorla Observatory. All the telescopes were equipped with CCD cameras and Johnson-Cousins $BVR_{\rm c}I_{\rm c}$ filters. Details on each telescope and observing procedures can be found in Villata et al. (1997), Tosti et al. (1996), Massaro et al. (1996) and Katajainen et al. (1997). Data reduction was performed with the standard IRAF, MIDAS, and other locally developed procedures such as Robin (see Villata et al. 1997) and REDUCE, the automatic reduction software developed by the Perugia group (Fiorucci & Tosti 1996). The comparison among the data acquired with different telescopes in the same night is fully satisfying: the mean differences are always less than one standard deviation, while the maximum differences do not exceed two standard deviations. Furthermore, no significant colour effect was found.

The $V,R_{\rm c},I_{\rm c}$ magnitudes of the comparison stars in the field of ON 231, used by all the groups to reduce the data, have been published by Fiorucci & Tosti (1996a). We extend now this sequence also to the B band; the B magnitudes of stars A, D, and C1 (see the finding chart reported by Fiorucci & Tosti 1996a) are: $12.64 \pm 0.03$ (star A), $16.37 \pm 0.05$ (star D), and $17.71\pm 0.05$ (star C1). The B values derived for the stars A and D are in agreement with the ones reported by Wing (1973).