  It has been recently proposed that the ejected matter from white dwarf (WD)
binary mergers can produce transient, optical and infrared emission similar to
the "kilonovae" of neutron star (NS) binary mergers. To confirm this we
calculate the electromagnetic emission from WD-WD mergers and compare with
kilonova observations. We simulate WD-WD mergers leading to a massive, fast
rotating, highly magnetized WD with an adapted version of the
smoothed-particle-hydrodynamics (SPH) code Phantom. We thus obtain initial
conditions for the ejecta such as escape velocity, mass and initial position
and distribution. The subsequent thermal and dynamical evolution of the ejecta
is obtained by integrating the energy-conservation equation accounting for
expansion cooling and a heating source given by the fallback accretion onto the
newly-formed WD and its magneto-dipole radiation. We show that magnetospheric
processes in the merger can lead to a prompt, short gamma-ray emission of up to
$\approx 10^{46}$ erg in a timescale of $0.1$-$1$ s. The bulk of the ejecta
initially expands non-relativistically with velocity $0.01 c$ and then it
accelerates to $0.1 c$ due to the injection of fallback accretion energy. The
ejecta become transparent at optical wavelengths around $\sim 7$ days
post-merger with a luminosity $10^{41}$-$10^{42}$ erg s$^{-1}$. The X-ray
emission from the fallback accretion becomes visible around $\sim 150$-$200$
day post-merger with a luminosity of $10^{39}$ erg s$^{-1}$. We also predict
the post-merger time at which the central WD should appear as a pulsar
depending on the value of the magnetic field and rotation period.
