UBV PHOTOELECTRIC PHOTOMETRY OF RED GIANTS IN NGC-6939

UBV photoelectric photometry has been obtained for 37 red giant stars in the intermediate-age open cluster NGC 6939 to study the morphology of the red giant branch in the colour-magnitude diagram. NGC 6939 presents a prominent clump. A second and fainter concentration is observed and should correspond to the slow-down of the evolution along the red giant branch occurring when the H-burning shell reaches the chemical composition discontinuity. Comparisons with isochrones and theoretical distribution functions along the red giant branch are not fully satisfactory. The canonical models from Castellani et al. (1992) produce a good agreement with the slope of the giant branch and width of the clump, but predict too many red giants in the lower part of the ascending giant branch, while the models with overshooting of Bressan et al. (1993) and Schaller et al. (1992) give probably better predictions.