1999A&A...343..825C

We report on near IR (J and K bands) observations of two 8x8(arcmin)² regions centered on the old open clusters Berkeley 17 and Berkeley 18, for which only optical photometry (in B,V and I bands) exists. J and K photometry allows us to obtain an independent estimate of cluster metallicity by means of the relationship between the spectroscopic metallicity and the Red Giant Branch (RGB) slope calibrated by Tiede et al. (1997AJ....114..694T). From the analysis of the colour magnitude diagram (CMD) and luminosity function (LF), Berkeley 17 turns out to have a metal content [Fe/H]~-0.35. It is 9Gyr old, suffers from a reddening E(B-V)=0.58mag and has an heliocentric distance of 2.5 kpc. Berkeley 17 comes out to be substantially younger than in previous work (age≃13Gyr). On the other hand Berkeley 18 is found to have solar metal abundance, and to be younger than Berkeley 17, with an age of about 4Gyr. While we confirm Kaluzny (1997A&AS..121..455K) reddening estimate, we significantly revise the distance of the cluster, which lies 4.5kpc from the Sun. These results on two open clusters believed to be between the oldest put constraints on the age and the evolution of the Galactic Disc. The absence of clusters older than 8-9Gyr suggests the possibility that the Galaxy underwent a star formation minimum between 13 and 10Gyr ago.