  We present our findings on the spectral analysis of seven magnetic white
dwarfs that were presumed to be double degenerates. We obtained time-resolved
spectroscopy at the Gemini Observatory to look for evidence of binarity or fast
rotation. We find three of our targets have rotation periods of less than an
hour based on the shifting positions of the Zeeman-split H$\alpha$ components:
13, 35, and 39 min, and we find one more target with a ~hour long period that
is currently unconstrained. We use offset dipole models to determine the
inclination, magnetic field strength, and dipole offset of each target. The
average surface field strengths of our fast rotators vary by 1-2 MG between
different spectra. In all cases, the observed absorption features are too
shallow compared to our models. This could be due to extra flux from a
companion for our three low-mass targets, but the majority of our sample likely
requires an inhomogeneous surface composition. Including an additional magnetic
white dwarf with similar properties presented in the literature, we find that 5
of the 8 targets in this sample show field variations on minute/hour
timescales. A crystallization driven dynamo can potentially explain the
magnetic fields in three of our targets with masses above $0.7~M_{\odot}$ but
another mechanism is still needed to explain their rapid rotation. We suggest
that rapid rotation or low-masses point to binary evolution as the likely
source of magnetism in 7 of these 8 targets.

Solar and Stellar Astrophysics