The Hubble Space Telescope UV Legacy Survey of Galactic Globular
  Clusters. XXI. Binaries among multiple stellar populations
  A number of scenarios for the formation of multiple populations in globular
clusters (GCs) predict that second generation (2G) stars form in a compact and
dense subsystem embedded in a more extended first-generation (1G) system. If
these scenarios are accurate, a consequence of the denser 2G formation
environment is that 2G binaries should be more significantly affected by
stellar interactions and disrupted at a larger rate than 1G binaries. The
fractions and properties of binary stars can thus provide a dynamical
fingerprint of the formation epoch of multiple-population GCs and their
subsequent dynamical evolution. We investigate the connection between binaries
and multiple populations in five GCs, NGC 288, NGC 6121 (M 4), NGC 6352, NGC
6362, and NGC 6838 (M 71). To do this, we introduce a new method based on the
comparison of Hubble Space Telescope observations of binaries in the F275W,
F336W, F438W, F606W and F814W filters with a large number of simulated
binaries. In the inner regions probed by our data we do not find large
differences between the local 1G and the 2G binary incidences in four of the
studied clusters, the only exception being M 4 where the 1G binary incidence is
about three times larger than the 2G incidence. The results found are in
general agreement with the results of simulations predicting significant
differences in the global 1G and 2G incidences and in the local values in the
clusters' outer regions but similar incidences in the inner regions. The
significant difference found in M 4 is consistent with simulations with a
larger fraction of wider binaries. Our analysis also provides the first
evidence of mixed (1G-2G) binaries, a population predicted by numerical
simulations to form in a cluster's inner regions as a result of stellar
encounters during which one component of a binary is replaced by a star of a
different population.
