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A crucial element in modelling the evolution of globular clusters is knowledge of their relative population of close binary systems. Such estimates are difficult to make using ground-based observations because of stellar crowding, especially near the cores of rich clusters where the stellar separation is typically less than 0.4 arcsec. We present results based on HST PC images which overcome this problem, providing differential time series photometry, in the U band, of some 20000 stars in a $66\times66$ arcsec field centred on the core of 47 Tucanae. The photon-noise limited time series, characterised by an exposure length of 1000s and continuous observing over 39 hours are ideal for detecting W Ursae Majoris systems. We discuss the W UMa systems found with HST and, using Monte Carlo simulations, estimate the frequency of these systems in the core of 47 Tuc.
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