  In this paper, we present a model for the long-term evolution of the merger
of two unequal mass C/O white dwarfs (WDs). After the dynamical phase of the
merger, magnetic stresses rapidly redistribute angular momentum, leading to
nearly solid-body rotation on a viscous timescale of 1e-4 to 1 yr, long before
significant cooling can occur. Due to heating during the dynamical and viscous
phases, the less massive WD is transformed into a hot, slowly rotating, and
radially extended envelope supported by thermal pressure.
  Following the viscous phase of evolution, the maximum temperature near the
envelope base may already be high enough to begin off-center convective
carbon-burning. If not, Kelvin-Helmholtz contraction of the inner region of the
envelope on a thermal timescale of 1e3-1e4 yr compresses the base of the
envelope, again yielding off-center burning. As a result, the long-term
evolution of the merger remnant is similar to that seen in previous
calculations: the burning shell diffuses inwards over ~1e4 yr, eventually
yielding a high-mass O/Ne WD or a collapse to a neutron star. During the
cooling and shell-burning phases, the merger remnant radiates near the
Eddington limit. Given the double WD merger rate of a few per 1000 yr, tens of
these ~1e38 erg/s sources should exist in a Milky Way-type galaxy.
  While the end result is similar to that of previous studies, the physical
picture and the dynamical state of the matter in our model differ from previous
work. Furthermore, remaining uncertainties related to the convective structure
near the photosphere and mass loss during the thermal evolution may
significantly affect our conclusions. Thus, future work within the context of
the physical model presented here is required to better address the eventual
fate of double WD mergers, including those for which one or both of the
components is a He WD.
