UBV photometric study and basic parameters of the southern open cluster NGC 2539

We present UBV photoelectric observations of 345 stars in the field of the southern open cluster NGC 2539. The analysis of these data allows to determ ine that 169 stars are probable members of the cluster main sequence, while 23 are possible members. The CC and CM diagrams reveal a well-defined main sequence and a populous red giant branch. We derive a reddening E(B - V) = 0.06 and an apparent distance modulus V - M-v = 10.60, equivalent to a dis tance of 1210 pc. The age, determined by fitting isochrones computed by the Geneva group with mass loss and moderate core overshooting, turns out to b e 630 Myr, which places this cluster within the Hyades-age group. The isoch rone for log t = 8.80 reproduces well the morphology of the upper main sequ ence band in the two CM dia,srams, including the binary ridge. Although thi s isochrone also reproduces well the general shape of the observed red gian t pattern, it appears to be a little too bright and too red. This fact coul d be probably due to the uncertainty on the exact value of the mixing-lengt h parameter. However, mass;loss during the evolution of the red giants migh t also partially account for their location in the HR diagram. The low cont amination of the upper main sequence of the cluster CM diagrams and its pop ulated red giant branch makes NGC 2539 a very good target for testing of th eoretical models.