  We present the result of a detailed analysis of HST UV and optical deep
images of the massive and young (~1.5 Gyr) stellar cluster NGC 1783 in the
Large Magellanic Cloud. This system does not show evidences of multiple
populations (MPs) along the red giant branch (RGB) stars. However, we find that
the cluster main-sequence (MS) shows evidence of a significant broadening (50%
larger than what is expected from photometric errors) along with hints of
possible bimodality in the MP sensitive (F343N - F438W, F438W)
color-magnitude-diagram (CMD). Such an effect is observed in all color
combinations including the F343N filter, while it is not found in the optical
CMDs. This observational evidence suggests we might have found light-element
chemical abundance variations along the MS of NGC 1783, which represents the
first detection of MPs in a system younger than 2 Gyr. A comparison with
isochrones including MP-like abundances shows that the observed broadening is
compatible with a N abundance enhancement of Delta([N/Fe]) ~0.3. Our analysis
also confirms previous results about the lack of MPs along the cluster RGB.
However, we find that the apparent disagreement between the results found on
the MS and the RGB is compatible with the mixing effects linked to the first
dredge-up. This study provides new key information about the MP phenomenon and
suggests that star clusters form in a similar way at any cosmic age.
