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The binary fraction in the globular cluster NGC 6397

Using Hubble Space Telescope (HST) observations of the globular cluster NGC6397, we constrain the cluster's binary fraction. The observations consist of two fields: the primary science field, a single ACS pointing centered approximately 5' from the cluster center; and the parallel field, a single WFPC2 field centered on the cluster center. Using the exquisite photometric precision of these observations, we determine the binary fraction in these regions of the cluster by examining stars lying off the main sequence. The binary fraction is constrained to be 0.012 +/- 0.004 in the ACS field, and to be 0.051 +/- 0.010 in the WFPC field. N-body simulations by Hurley et al. (2007) suggest that the binary fraction remains nearly constant beyond the half-mass radius for the lifetime of the cluster. In the context of these simulations, our results suggest that NGC 6397 had a primordial binary fraction of only ~1%.