  Merger of a white dwarf binary creates a differentially rotating object which
is expected to generate strong magnetic fields. Kinetic energy stored in
differential rotation is partially dissipated in the magnetically dominated
corona, which forms a hot variable outflow with ejection velocity comparable to
$10^9$ cm s$^{-1}$. The outflow should carry significant mass and energy for
hours to days, creating an expanding fireball with the following features. (i)
The fireball is initially opaque and its internal energy is dominated by the
trapped thermal radiation. The stored heat is partially converted to kinetic
energy of the flow (through adiabatic cooling) and partially radiated away.
(ii) Internal shocks develop in the fireball and increase its radiative output.
(iii) A significant fraction of the emitted energy is in the optical band. As a
result, a bright optical transient with luminosity $L\sim 10^{41}-10^{42}$ erg
s$^{-1}$ and a characteristic peak duration comparable to 1 day may be expected
from the merger. In contrast to classical novae or supernovae, the transient
does not involve nuclear energy. The decay after its peak reflects the damping
of differential rotation in the merger remnant. Such outbursts may be detected
in the local Universe with current and upcoming optical surveys.
