NGC 1866 revisited: semiconvection versus overshoot.

We calculate a new set of stellar tracks and related isochrones that take into account the presence of semiconvection in the He-core burning phase. The stellar masses range from 2.5 to 9 M_sun_ for two values of the metallicity: Z = 0.02 and Z = 0.003. These models are compared with a set calculated with convective overshoot where the metallicity, the He content and the other parameters are homogeneous. Since the treatment of the convection in the core of a real star is affected by many uncertainties, we look at the observations of the star clusters of the Large Magellanic Cloud to discriminate among semiconvection and overshoot. In particular, we compare the theoretical models with the colour-magnitude diagram and the luminosity function of NGC 1866 previously presented by Chiosi et al. (1989). Both models reproduce the features of the colour-magnitude diagram; however only the convective overshoot hypothesis is in agreement with the observed ratio of main sequence to evolved stars.