The brighter stars in each cluster under discussion show
evolutionary effects. Most of them are confirmed proper motion cluster
members. The ages of these stars have been determined by fitting the
theoretical stellar evolutionary isochrones given by Bertelli et al.
(1994) in the corresponding CM diagrams
(Figs. 13 (click here)-16 (click here)). The isochrones are for Pop I stars
(X=0.7, Y=0.28 and Z=0.02) and include the effects of convective core
overshooting in the models. The isochrone fitting to the evolved MS, and to
the few bright red giants in NGC 5662 and NGC 6087, indicates that bright
stars have mean ages of 100 Myr in NGC 3228; of 30 Myr in NGC 4103; of 80
Myr in NGC 5662 and of 65 Myr in NGC 6087. In the case of NGC 3228, the age
is estimated only from the ,
and
,
diagrams
(Fig. 13 (click here)); as the brighter evolved cluster members do not have R
and I magnitudes.
The present age estimate for NGC 3228 is lower than the 250 Myr age estimated by Hogg (1963) but in good agreement with the value of 130 Myr given in Lyngå's (1987) catalogue. For NGC 4103, our age estimate is almost the same as given by Wesselink (1969). However, it is slightly lower than the value of 40 Myr given in Lyngå's (1987) catalogue. The age of NGC 5662 derived by us is in good agreement with the values of 60, 80 and 79 Myr given by Haug (1978), Lyngå\ (1987) and Clariá et al. (1991) respectively. Similarly, our age estimate for NGC 6087 is in good agreement with the values of 50 and 70 Myr determined by Turner (1986) and Lyngå (1987) respectively.