The dense central region of NGC 6397 contains three classes of stars whose origins are likely related to stellar interactions : blue stragglers ( BSs ) , cataclysmic variables ( CVs ) , and probable helium white dwarfs ( HeWDs ) .
We summarize results to date concerning CVs and HeWD candidates that have been identified in two imaging studies with Hubble Space Telescope ( HST ) , and present one new CV candidate that appears well outside the cluster core .
We also present results concerning binaries containing two main sequence stars in the central parts of the cluster .
Proper motion information derived from two epochs of HST data is used to remove field stars from the sample .
Binaries are then identified on the basis of their positions in the color-magnitude diagram .
We set an upper limit of \sim 3 % on the fraction of main sequence stars with primary masses in the range 0.45 - 0.8 { \cal M } _ { \odot } and mass ratios q \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 0.45 .
Extrapolating to all mass ratios gives an estimated binary fraction of \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 5 - 7 % .
Even in these small numbers , such pairs are likely to be key players in the processes that give rise to the more exotic stellar populations .