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The Chandra X-ray Observatory has detected relatively hard X-ray emissionfrom the central stars of several planetary nebulae (PNe). A subset have noknown late-type companions, making it very difficult to isolate which ofseveral competing mechanisms may be producing the X-ray emission. The centralstar of NGC 2392 is one of the most vexing members, with substantial indirectevidence for a hot white dwarf (WD) companion. Here we report on the results ofa radial velocity (RV) monitoring campaign of its central star with the HERMESechelle spectrograph of the Flemish 1.2 m Mercator telescope. We discover asingle-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 highdegree of nebula ionisation requires a WD companion ($M\gtrsim0.6 M_\odot$),which the mass-function supports at orbital inclinations $\lesssim$7 deg, inagreement with the nebula orientation of 9 deg. The hard component of the X-rayspectrum may be explained by the companion accreting mass from the wind of theRoche lobe filling primary, while the softer component may be due to collidingwinds. A companion with a stronger wind than the primary could produce thelatter and would be consistent with models of the observed diffuse X-rayemission detected in the nebula. The diffuse X-rays may also be powered by thejets of up to 180 km/s and active accretion would imply that they could be thefirst active jets of a post-common-envelope PN, potentially making NGC 2392 aninvaluable laboratory to study jet formation physics. The 1.9 d orbital periodrules out a double-degenerate merger leading to a Type Ia supernova and theweak wind of the primary likely also precludes a single-degenerate scenario. Wesuggest that a hard X-ray spectrum, in the absence of a late-type companion,could be a powerful tool to identify accreting WD companions.

The post-common-envelope X-ray binary nucleus of the planetary nebula NGC 2392