  We suggest that fast-rising blue optical transients (FBOTs) and the brightest
event of the class AT2018cow result from an electron-capture collapse to a \NS\
following a merger of a massive ONeMg white dwarf (WD) with another WD. Two
distinct evolutionary channels lead to the disruption of the less massive WD
during the merger and the formation of a shell burning non-degenerate star
incorporating the ONeMg core. During the shell burning stage a large fraction
of the envelope is lost to the wind, while mass and angular momentum are added
to the core. As a result, the electron-capture collapse occurs with a small
envelope mass, after $\sim 10^2-10^4$ years. During the formation of a neutron
star as little as $\sim 10^{-2} M_\odot $ of the material is ejected at the
bounce-off with mildly relativistic velocities and total energy $\sim$ few $
10^{50}$ ergs. This ejecta becomes optically thin on a time scale of days -
this is the FBOT. During the collapse, the neutron star is spun up and magnetic
field is amplified. The ensuing fast magnetically-dominated relativistic wind
from the newly formed neutron star shocks against the ejecta, and later against
the wind. The radiation-dominated forward shock produces the long-lasting
optical afterglow, while the termination shock of the relativistic wind
produces the high energy emission in a manner similar to Pulsar Wind Nebulae.
If the secondary WD was of the DA type, the wind will likely have $\sim 10^{-4}
M_\odot$ of hydrogen; this explains the appearance of hydrogen late in the
afterglow spectrum. The model explains many of the puzzling properties of
FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a
bright optical peak, afterglow high energy emission of similar luminosity to
the optical, and late infra-red features.
