  We report a peculiar variable blue star in the globular cluster NGC 6397,
using Hubble Space Telescope optical imaging. Its position in the
colour-magnitude diagrams, and its spectrum, are consistent with this star
being a helium core white dwarf (He WD) in a binary system. The optical light
curve shows a periodicity at 18.5 hours. We argue that this periodicity is due
to the rotation of the WD and possibly due to magnetic spots on the surface of
the WD. This would make this object the first candidate magnetic He WD in any
globular cluster (GC), and the first candidate magnetic WD in a detached binary
system in any GC and one of the few He WDs with a known rotation period and of
magnetic nature. Another possibility is that this system is a He WD in a binary
system with another WD or another degenerate object, which would make this
object one of the few candidate non-accreting double degenerate binaries in any
GC.
