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One of the most unexpected results in the field of stellar populations of thelast few years, is the discovery that some Magellanic-Cloud globular clustersyounger than ~400 Myr, exhibit bimodal main sequences (MSs) in theircolor-magnitude diagrams (CMDs). Moreover, these young clusters host anextended main sequence turn off (eMSTO) in close analogy with what is observedin most ~1-2 Gyr old clusters of both Magellanic Clouds.  We use high-precision Hubble-Space-Telescope photometry to study the youngstar cluster NGC1866 in the Large Magellanic Cloud. We discover an eMSTO and asplit MS. The analysis of the CMD reveals that (i) the blue MS is the lesspopulous one, hosting about one-third of the total number of MS stars; (ii)red-MS stars are more centrally concentrated than blue-MS stars; (iii) thefraction of blue-MS stars with respect to the total number of MS stars drops bya factor of ~2 in the upper MS with F814W <~19.7.  The comparison between the observed CMDs and stellar models reveals that theobservations are consistent with ~200 Myr old highly-rotating stars on thered-MS, with rotation close to critical value, plus a non-rotating stellarpopulation spanning an age interval between ~140 and 220 Myr, on the blue-MS.Noticeable, neither stellar populations with different ages only, nor coevalstellar models with different rotation rates, properly reproduce the observedsplit MS and eMSTO. We discuss these results in the context of the eMSTO andmultiple MS phenomenon.

Multiple stellar populations in Magellanic Cloud clusters. V. The split main sequence of the young cluster NGC1866