Testing Magnetic Braking: The X-ray Activity of M Dwarfs in Pre-Cataclysmic Variable Systems

Angular momentum loss by magnetic braking is believed to be an important mechanism in the evolution of tidally-locked white dwarf (WD) bina- ries. We measure the X-ray emission of a sample of M dwarfs in pre-cataclysmic variable systems to examine for the first time, if the magnetic activity of sec- ondary stars in tidally-locked white dwarf binaries is the same as that of their sin- gle main-sequence counterparts. Our results support the extrapolation of mag- netic braking prescriptions derived from single main-sequence stars to tidally- locked WD binaries, and therefore support recent results that indicate magnetic braking plays a smaller role than previously proposed.