THE LUMINOSITY FUNCTION OF STARS IN THE INTERMEDIATE AGE OPEN CLUSTERNGC-7789

We report the results of a CCD investigation of stars in a selected re gion near the center of the intermediate age galactic cluster NGC 7789 . A CM diagram is presented for a sample of 1122 stars, which should b e complete down to V = 20. On the ground of recent theoretical evoluti onary models, simulations of both the colour-magnitude diagram and the cluster luminosity function are discussed under different assumptions about age, mass function and binary population. We find that the fain t main sequence (V < 15) appears rather depopulated of stars, with a d istribution of luminosity lower than that expected in the case of a Sa lpeter's mass function. The distribution of post main sequence stars i n the various evolutionary phases is well reproduced by classical mode ls (i.e. with negligible overshooting of the unstable convective layer s). Data are best fitted by models having an age of 1.2 Gyr. Derived v alues for distance modulus and reddening are in agreement with previou s evaluations. We obtain V - M(v) = 12.3 +/- 0.2 and E(B - V) = 0.32 /- 0.03.