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We make use of the largest and most homogeneous sample of white dwarf/M dwarf(WD/dM) binaries from the Sloan Digital Sky Survey (SDSS DR7) to investigaterelations between magnetic activity, rotation, magnetic braking and age in Mstars. These relations are studied separately for close WD/dM binaries thatunderwent a common envelope phase and thus contain tidally locked and hencerapidly rotating M dwarfs, and for wide WD/dM binaries that never interacted.For the wide WD/dM binary sample we find that the M dwarf activity fractionsare significantly higher than those measured in single M stars of spectral typeM0 to M5. We attribute this effect as a consequence of M dwarfs in wide SDSSWD/dM binaries being, on average, significantly younger and hence more activethan the field M dwarf population. The measured M dwarf activity fractions inwide WD/dM binaries show as well a significant increase from spectral types M3to M5, where these low-mass stars become fully convective. This providesadditional observational evidence for magnetic braking being less efficientbelow the fully convective boundary, in agreement with the hypothesis of fullyconvective stars having considerably longer activity lifetimes than partiallyconvective stars. The M dwarfs in all our close binaries are active,independently of the spectral type, giving robust observational evidence formagnetic activity being enhanced due to fast rotation. The rotationalvelocities of the M dwarfs in our close binary sample are significantly higherthan seen among field M dwarfs, however the strength of magnetic activityremains saturated at log LHalpha/Lbol approximately -3.5. This unambiguouslyconfirms the M dwarf saturation-type rotation-activity relation.

M dwarf companions to white dwarfs I: relating magnetic activity, rotation and age