  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%.
