A complete census of $H\alpha$ emitters in NGC 6397
  We used a dataset of archival Hubble Space Telescope images obtained through
the F555W, F814W and F656N filters, to perform a complete search for objects
showing $H\alpha$ emission in the globular cluster NGC 6397. As photometric
diagnostic, we used the $(V-H\alpha)_0$ color excess in the
$(V-H\alpha)_0$-$(V-I)_0$ color-color diagram. In the analysed field of view,
we identified 53 $H\alpha$ emitters. In particular, we confirmed the optical
counterpart to 20 X-ray sources (7 cataclysmic variables, 2 millisecond pulsars
and 11 active binaries) and identified 33 previously unknown sources, thus
significantly enlarging the population of known active binaries in this
cluster. We report the main characteristics for each class of objects.
Photometric estimates of the equivalent width of the $H\alpha$ emission line,
were derived from the $(V-H\alpha)_0$-excess and, for the first time, compared
to the spectroscopic measurements obtained from the analysis of MUSE spectra.
The very good agreement between the spectroscopic and photometric measures
fully confirmed the reliability of the proposed approach to measure the
$H\alpha$ emission. The search demonstrated the efficiency of this novel
approach to pinpoint and measure $H\alpha$-emitters, thus offering a powerful
tool to conduct complete census of objects whose formation and evolution can be
strongly affected by dynamical interactions in star clusters.
