Near IR photometry of the old open clusters Berkeley 17 and Berkeley 18 - Probing the age of the Galactic Disc

We report on near IR (J and K bands) observations of two 8 x 8 (arcmin)(2) regions centered on the old open clusters Berkeley 17 and Berkeley 18, for which only optical photometry tin B,V and I bands) exists. J and K photomet ry allows us to obtain an independent estimate of cluster metallicity by me ans of the relationship between the spectroscopic metallicity and the Red G iant Branch (RGB) slope calibrated by Tiede et al. (1997). From the analysis of the colour magnitude diagram (CMD) and luminosity func tion (LF), Berkeley 17 turns out to have a metal content [Fe/H] similar to - 0.35. It is 9 Gyr old, suffers from a reddening E(B - V) = 0.58 mag and h as an heliocentric distance of 2.5 kpc. Berkeley 17 comes out to be substan tially younger than in previous work (age approximate to 13 Gyr). On the other hand Berkeley 18 is found to have solar metal abundance, and t o be younger than Berkeley 17, with an age of about 4 Gyr. While we confirm Kaluzny (1997) reddening estimate, we significantly revise the distance of the cluster, which lies 4.5 kpc from the Sun. These results on two open cl usters believed to be between the oldest put constraints on the age and the evolution of the Galactic Disc. The absence of clusters older than 8-9 Gyr suggests the possibility that the Galaxy underwent a star formation minimu m between 13 and 10 Gyr ago.