An ACS Deep Field in the Globular Cluster NGC 6397: Variability and Binary Frequency

In March 2005, 126 orbits of the Hubble Space Telescope were dedicated to obtaining extremely deep images of the globular cluster NGC6397. Two fields were simultaneously imaged: the core of the cluster was imaged with the Wide Field and Planetary Camera II (WPFC2), and a field 5 arcminutes from the core was imaged with the Advanced Camera for Surveys (ACS). Although the primary science driver for obtaining these data was the study of ultra-cool white dwarfs, the time sampling of these images made them well suited for the search and study of low-amplitude, faint stellar variability -- primarily due to binarity. The recently obtained fields are coincident with archival WFPC2 data. This allows a very clean determination of cluster membership independent of photometric measurements. Here we report on the search for binary star systems via their photometric variability in both the ACS and the WPFC2 fields.