Young star clusters in the Large Magellanic Cloud: NGC 1805 and 1818

We present colour-magnitude diagrams for two rich (approximate to 10(4) M.) Large Magellanic Cloud star clusters with ages approximate to 10(7) yr, constructed from optical and near-infrared data obtained with the Hubble Space Telescope. These data are part of an HST project to study LMC clusters with a range of ages. In this paper we investigate the massive star content of the young clusters, and determine the cluster ages and metallicities, paying particular attention to Be-star and blue-straggler populations and evidence of age spreads. We compare our data with detailed stellar-population simulations to investigate the turn-off structure of approximate to 25 Myr stellar systems, highlighting the complexity of the blue-straggler phenomenon.