  White dwarf (WD) binary mergers are possible progenitors to a number of
unusual stars and transient phenomena, including type Ia supernovae. To date,
simulations of mergers have not included magnetic fields, even though they are
believed to play a significant role in the evolution of the merger remnant. We
simulated a 0.625 - 0.65 $M_{\odot}$ carbon-oxygen WD binary merger in the
magnetohydrodynamic moving mesh code Arepo. Each WD was given an initial dipole
field with a surface value of $\sim10^3$ G. As in simulations of merging double
neutron star binaries, we find exponential field growth within Kelvin-Helmholtz
instability-generated vortices during the coalescence of the two stars. The
final field has complex geometry, and a strength $>10^{10}$ G at the center of
the merger remnant. Its energy is $\sim2\times10^{47}$ ergs, $\sim0.2$% of the
remnant's total energy. The strong field likely influences further evolution of
the merger remnant by providing a mechanism for angular momentum transfer and
additional heating, potentially helping to ignite carbon fusion.
