We present a study of the central regions of NGC 6397 using HST/ACS,
focusing on a group of ~24 faint blue stars that form a sequence
parallel to, but brighter than, the sequence of carbon-oxygen white
dwarfs. We show that these stars, 18 of which are newly discovered,
have magnitudes and colors consistent with those of helium-core WDs
with masses ~0.2-0.3 Msun. Their H-alpha - R625 colors indicate that
they have strong H-alpha absorption lines, which distinguishes them
from cluster CVs. The radial distribution of the He WDs is
significantly more concentrated than that of either the CO WDs or the
turnoff stars. Binary companions are required to explain the implied
dynamical masses of >0.8 Msun. We show that the companions are most
likely heavy CO WDs and set constraints on the number and
characteristics of the cluster binaries needed to explain the He WDs.
In contrast to the CO WD sequence, the He WD sequence ends well above
the magnitude limit of the observations. We explore the significance
of this finding in the context of thick vs. thin hydrogen envelope
models and compare our results to existing theoretical predictions.
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