MULTICOLOR NTT CCD PHOTOMETRY OF THE POST-CORE-COLLAPSE GLOBULAR-CLUSTER NGC-6397

We have carried out multicolor (UBVI) CCD photometry for more than 250 0 stars (12(m) < V < 22(m)) in the field of the nearby globular cluste r NGC 6397. We find the main-sequence turnoff at V-TO = 16.45 +/- 0.05 , (B-V)(TO) = 0.550 +/- 0.012, (V-I)(TO) = 0.790 +/- 0.010, and (B-I)( TO) = 1.340 +/- 0.015. The reddening value is found to be E(B-V) = 0.1 7 +/- 0.04. Our comparison of the color-magnitude diagrams with isochr ones from VandenBerg & Bell (1985, ApJS, 58, 561) and Bergbusch & Vand enBerg (1992, ApJS, 81, 163) shows that the cluster's age somewhat exc eeds 16 Gyr in the scale of Bergbusch & VandenBerg isochrones and 18 G yr in the scale of VandenBerg & Bell isochrones. The mean main-sequenc e locus for NGC 6397 in the V-(B-V) and V-(B-I) diagrams is well repro duced by both sets of isochrones in a wide magnitude range. We estimat e the upper limit for binaries population in the studied cluster field , at r similar to 8' from the cluster center, as similar to 9%. We hav e compared the main sequence luminosity functions for NGC 6397 and M4 (Alcaino et al. 1997, AJ, 114, 189). For NGC 6397, we find a luminosit y function depression at V = 18.0, similar to that noticed for M4 at V = 18.6 but less significant. (C) 1997 American Astronomical Society.