8 The mean reddening, distance and age

Using the extinction map derived in Sect. 7, we corrected the CM diagram for the differential reddening. There is an advantage in using such a CM diagram for the derivation of cluster parameters, especially its age. The problem has already been pointed out by Phelps & Janes (1994): a large spread in colours of stars close to the turn-off point led them to the conclusion that the possible range of age of NGC663 is quite wide, between 12 and 25 Myr. An age of about 10 Myr was also proposed by Liu et al. (1991) and Tapia et al. (1991), while Mermilliod & Maeder (1983) include NGC663 in the group of clusters with an age of about 21 Myr.

We used our CM diagram corrected for differential reddening for the estimation of age, true distance modulus and the mean reddening of NGC663. For this purpose we applied the isochrones of Bertelli et al. (1994) for Y = 0.28 and Z = 0.02. The results of fitting the isochrones are shown in Fig. 8. We finally adopt 11.6 mag as the true distance modulus of NGC663 (it corresponds to a distance of 2.1 kpc). This agrees very well with the recent determinations of the distance modulus given by others: 11.80 mag (Moffat 1972), 11.55 mag (van den Bergh & de Roux 1978), 12.03 $\pm$ 0.20 mag (Tapia et al. 1991), 12.25 mag (Phelps & Janes 1994), and 11.6 $\pm$ 0.7 (Fabregat & Torrejón 2000). The mean cluster colour-excess is $E(R-I)_{\rm C}$ = 0.54 mag. This corresponds to $E(B-V) \approx$0.83 mag, in good agreement with other determinations (Johnson et al. 1961; Hoag 1965; Moffat 1972; Phelps & Janes 1994). In our fit we assumed the ratio of total to selective absorption $A_{\rm Rc}/E(R-I)_{\rm C}$ = 3.7.

As can be seen in Fig. 8, the cluster upper main sequence is best fitted by the $\log$ (time in years) = 7.3 and 7.4 isochrones. This results in the cluster age in the range between 20 and 25 Myr.