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, co nstructed from optical and near-infrared data obtained with the Hubble Spac e Telescope. These data are part of an HST project to study LMC clusters wi th a range of ages. In this paper we investigate the massive star content o f the young clusters, and determine the cluster ages and metallicities, pay ing particular attention to Be-star and blue-straggler populations and evid ence of age spreads. We compare our data with detailed stellar-population s imulations to investigate the turn-off structure of approximate to 25 Myr s tellar systems, highlighting the complexity of the blue-straggler phenomeno n.