The Cepheids of NGC1866: A Precise Benchmark for the Extragalactic
  Distance Scale and Stellar Evolution from Modern UBVI Photometry
  We present the analysis of multiband time-series data for a sample of 24
Cepheids in the field of the Large Magellanic Cloud cluster NGC1866. Very
accurate BVI VLT photometry is combined with archival UBVI data, covering a
large temporal window, to obtain precise mean magnitudes and periods with
typical errors of 1-2% and of 1 ppm, respectively. These results represent the
first accurate and homogeneous dataset for a substantial sample of Cepheid
variables belonging to a cluster and hence sharing common distance, age and
original chemical composition. Comparisons of the resulting multiband
Period-Luminosity and Wesenheit relations to both empirical and theoretical
results for the Large Magellanic Cloud are presented and discussed to derive
the distance of the cluster and to constrain the mass-luminosity relation of
the Cepheids. The adopted theoretical scenario is also tested by comparison
with independent calibrations of the Cepheid Wesenheit zero point based on
trigonometric parallaxes and Baade-Wesselink techniques. Our analysis suggests
that a mild overshooting and/or a moderate mass loss can affect
intermediate-mass stellar evolution in this cluster and gives a distance
modulus of 18.50 +- 0.01 mag. The obtained V,I color-magnitude diagram is also
analysed and compared with both synthetic models and theoretical isochrones for
a range of ages and metallicities and for different efficiencies of core
overshooting. As a result, we find that the age of NGC1866 is about 140 Myr,
assuming Z = 0.008 and the mild efficiency of overshooting suggested by the
comparison with the pulsation models.
