  Merging white dwarfs are a possible progenitor of Type Ia supernovae (SNe
Ia). While it is not entirely clear if and when an explosion is triggered in
such systems, numerical models suggest that a detonation might be initiated
before the stars have coalesced to form a single compact object. Here we study
such "peri-merger" detonations by means of numerical simulations, modeling the
disruption and nucleosynthesis of the stars until the ejecta reach the coasting
phase. Synthetic light curves and spectra are generated for comparison with
observations. Three models are considered with primary masses 0.96 Msun, 1.06
Msun, and 1.20 Msun. Of these, the 0.96 Msun dwarf merging with an 0.81 Msun
companion, with a Ni56 yield of 0.58 Msun, is the most promising candidate for
reproducing common SNe Ia. The more massive mergers produce unusually luminous
SNe Ia with peak luminosities approaching those attributed to
"super-Chandrasekhar" mass SNe Ia. While the synthetic light curves and spectra
of some of the models resemble observed SNe Ia, the significant asymmetry of
the ejecta leads to large orientation effects. The peak bolometric luminosity
varies by more than a factor of 2 with the viewing angle, and the velocities of
the spectral absorption features are lower when observed from angles where the
light curve is brightest. The largest orientation effects are seen in the
ultraviolet, where the flux varies by more than an order of magnitude. Despite
the large variation with viewing angle, the set of three models roughly obeys a
width-luminosity relation, with the brighter light curves declining more slowly
in the B-band. Spectral features due to unburned carbon from the secondary star
are also seen in some cases.
