| Abstract: | We determine the binary star fraction as a function of radius in NGC 1818, a young rich cluster in the Large Magellanic Cloud, using Hubble Space Telescope images in bands F336W (～ U ) and F555W (～ V ). Our sample includes binaries with M primary ～ 2–5.5 M⊙ and M secondary ? 0.7 Mprimary. The binary fraction increases towards the cluster centre, from ～ 20 ± 5 per cent in the outer parts, to ～ 35 ± 5 per cent inside the core. This increase is consistent with dynamical mass segregation and need not be primordial. We compare our results with expectations from N -body models, and discuss the implications for the formation and early evolution of such clusters. |
