Analysis of colour-magnitude diagrams of rich LMC clusters

We present a preliminary analysis of a deep colour-magnitude diagram of NGC 1831, a rich cluster in the LMC. The data were obtained with HST/WFPC2 in the F555W and F814W filters, reaching m(555) similar or equal to 25 mag. Efficient use of the two-dimensional distribution of data was made by comparing the observed colour-magnitude diagram with model ones. We discuss the process used in accounting for field star contamination and sampling incompleteness in the data. We also describe the algorithm used in building artificial diagrams from model isochrones and the statistical method used in comparing model and data colour-magnitude diagrams. From these comparisons we infer a metallicity Z = 0.008 and a low value, f(bin) similar or equal to 0.25, for the global binary fraction of NGC 1831. We also rule out ages T less than or equal to 500 Myr and mass function slope values a greater than or equal to 2.