SIMBAD references

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 determine 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 distance of 1210pc. The age, determined by fitting isochrones computed by the Geneva group with mass loss and moderate core overshooting, turns out to be 630Myr, which places this cluster within the Hyades-age group. The isochrone for logt=8.80 reproduces well the morphology of the upper main sequence band in the two CM diagrams, including the binary ridge. Although this isochrone also reproduces well the general shape of the observed red giant pattern, it appears to be a little too bright and too red. This fact could be probably due to the uncertainty on the exact value of the mixing-length parameter. However, mass loss during the evolution of the red giants might also partially account for their location in the HR diagram. The low contamination of the upper main sequence of the cluster CM diagrams and its populated red giant branch makes NGC 2539 a very good target for testing of theoretical models.