  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 investigate
relations between magnetic activity, rotation, magnetic braking and age in M
stars. These relations are studied separately for close WD/dM binaries that
underwent a common envelope phase and thus contain tidally locked and hence
rapidly 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 fractions
are significantly higher than those measured in single M stars of spectral type
M0 to M5. We attribute this effect as a consequence of M dwarfs in wide SDSS
WD/dM binaries being, on average, significantly younger and hence more active
than the field M dwarf population. The measured M dwarf activity fractions in
wide WD/dM binaries show as well a significant increase from spectral types M3
to M5, where these low-mass stars become fully convective. This provides
additional observational evidence for magnetic braking being less efficient
below the fully convective boundary, in agreement with the hypothesis of fully
convective stars having considerably longer activity lifetimes than partially
convective stars. The M dwarfs in all our close binaries are active,
independently of the spectral type, giving robust observational evidence for
magnetic activity being enhanced due to fast rotation. The rotational
velocities of the M dwarfs in our close binary sample are significantly higher
than seen among field M dwarfs, however the strength of magnetic activity
remains saturated at log LHalpha/Lbol approximately -3.5. This unambiguously
confirms the M dwarf saturation-type rotation-activity relation.
