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We present a study of close white dwarf and M dwarf (WD+dM) binary systemsand examine the effect that a close companion has on the magnetic fieldgeneration in M dwarfs. We use a base sample of 1602 white dwarf -- mainsequence binaries from Rebassa et al. to develop a set of color cuts in GALEX,SDSS, UKIDSS, and 2MASS color space to construct a sample of 1756 WD+dMhigh-quality pairs from the SDSS DR8 spectroscopic database. We separate theindividual WD and dM from each spectrum using an iterative technique thatcompares the WD and dM components to best-fit templates. Using the absoluteheight above the Galactic plane as a proxy for age, and the H{\alpha} emissionline as an indicator for magnetic activity, we investigate the age-activityrelation for our sample for spectral types \leqM7. Our results show thatearly-type M dwarfs (\leqM4) in close binary systems are more likely to beactive and have longer activity lifetimes compared to their field counterparts.However, at a spectral type of M5 (just past the onset of full convection in Mdwarfs), the activity fraction and lifetimes of WD+dM binary systems becomesmore comparable to that of the field M dwarfs. One of the implications ofhaving a close binary companion is presumed to be increased stellar rotationthrough disk-disruption, tidal effects, or angular momentum exchange. Thus, weinterpret the similarity in activity behavior between late-type dMs in WD+dMpairs and late-type field dMs to be due to a decrease in sensitivity in closebinary companions (or stellar rotation), which has implications for the natureof magnetic activity in fully-convective stars. (Abridged)

The Effects of Close Companions (and Rotation) on the Magnetic Activity of M Dwarfs