Be star surveys with CCD photometry . II . NGC 1818 and its neighbouring cluster in the LMC ⋆

As part of an ongoing photometric survey of young Magellanic Cloud clusters we identified Be stars in NGC 1818 and a nearby smaller cluster in the Large Magellanic Cloud. The neighbouring cluster does not appear to contain any evolved stars, and its sparsely populated main sequence does not extend to stars as massive as in NGC 1818. Both clusters are younger than the surrounding field population, but the current data do not allow to conclude whether NGC 1818 is a binary cluster or not. The small cluster is more heavily reddened than NGC 1818 indicating the presence of differential reddening, leftover gas and dust from the star formation process, or a larger distance. NGC 1818 does not seem to be significantly affected by differential reddening. We find both clusters to be rich in Be stars. The field only contains very few Be stars as one would expect for a predominantly older population. NGC 1818 contains almost as many Be stars as the slightly younger SMC cluster NGC 330, while NGC 2004, a young LMC cluster, has a lower Be star content. We discuss problems in comparing Be star fractions in Magellanic Cloud clusters and Galactic open clusters and possible constraints on Be star theories.