  We present a new deep (down to V ~ 24) photometry of a wide region (6'x 6')
around the LMC globular cluster NGC1866: our sample is complete, down to 3 mag
below the brightest MS star. Detailed comparisons with various theoretical
scenarios using models computed with the evolutionary code FRANEC have been
done reaching the following conclusions: both standard models (i.e. computed by
adopting the Schwarzschild criterion to fix the border of the convective core)
and models with an enlarged convective core (overshooting) lead to a fair fit
of the MS but are not able to reproduce the luminosity and/or the number of He
burning giants. Models including a fraction of 30% of binaries leads to a good
fit both to the MS luminosity function and to the He clump, if standards models
are considered, for a visual distance modulus (m-M)v = 18.8, age t ~ 100 Myr
and mass function slope alpha ~ 2.4, thus largely removing the "classical"
discrepancy between observed and predicted number of stars in the He burning
clump. The fit obtained with models computed with an enlarged convective core
gets worse when a binary component is taken into account, because the presence
of binary systems increases the existing discrepancy between the observed and
predicted clump luminosity. As a consequence of this analysis, we conclude that
the next step towards a proper understanding of NGC 1866, and similar clusters,
must include the accurate determination of the frequency of binary systems that
will be hopefully performed with the incoming Cycle 8 HST observations of
NGC~1866.
