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As main-sequence stars with C$>$O, dwarf carbon (dC) stars are never bornalone but inherit carbon-enriched material from a former asymptotic giantbranch (AGB) companion. In contrast to M dwarfs in post-mass transfer binaries,C$_2$ and/or CN molecular bands allow dCs to be identified withmodest-resolution optical spectroscopy, even after the AGB remnant has cooledbeyond detectability. Accretion of substantial material from the AGB starsshould spin up the dCs, potentially causing a rejuvenation of activitydetectable in X-rays. Indeed, a few dozen dCs have recently been found to havephotometric variability with periods under a day. However, most of those arelikely post-common-envelope binaries (PCEBs), spin-orbit locked by tidalforces, rather than solely spun-up by accretion. Here, we study the X-rayproperties of a sample of the five nearest known dCs with $Chandra$. Two aredetected in X-rays, the only two for which we also detected short-periodphotometric variability. We suggest that the coronal activity detected so farin dCs is attributable to rapid rotation due to tidal locking in short binaryorbits after a common-envelope phase, late in the thermally pulsing (TP) phaseof the former C-AGB primary (TP-AGB).

New Clues to the Evolution of Dwarf Carbon Stars From Their Variability and X-ray Emission