  The Chandra X-ray Observatory has detected relatively hard X-ray emission
from the central stars of several planetary nebulae (PNe). A subset have no
known late-type companions, making it very difficult to isolate which of
several competing mechanisms may be producing the X-ray emission. The central
star of NGC 2392 is one of the most vexing members, with substantial indirect
evidence for a hot white dwarf (WD) companion. Here we report on the results of
a radial velocity (RV) monitoring campaign of its central star with the HERMES
echelle spectrograph of the Flemish 1.2 m Mercator telescope. We discover a
single-lined spectroscopic binary with an orbital period of
$1.902208\pm0.000013$ d and a RV semi-amplitude of $9.96\pm0.13$ km/s. The high
degree of nebula ionisation requires a WD companion ($M\gtrsim0.6 M_\odot$),
which the mass-function supports at orbital inclinations $\lesssim$7 deg, in
agreement with the nebula orientation of 9 deg. The hard component of the X-ray
spectrum may be explained by the companion accreting mass from the wind of the
Roche lobe filling primary, while the softer component may be due to colliding
winds. A companion with a stronger wind than the primary could produce the
latter and would be consistent with models of the observed diffuse X-ray
emission detected in the nebula. The diffuse X-rays may also be powered by the
jets of up to 180 km/s and active accretion would imply that they could be the
first active jets of a post-common-envelope PN, potentially making NGC 2392 an
invaluable laboratory to study jet formation physics. The 1.9 d orbital period
rules out a double-degenerate merger leading to a Type Ia supernova and the
weak wind of the primary likely also precludes a single-degenerate scenario. We
suggest that a hard X-ray spectrum, in the absence of a late-type companion,
could be a powerful tool to identify accreting WD companions.
