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We suggest that fast-rising blue optical transients (FBOTs) and the brightestevent 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. Twodistinct evolutionary channels lead to the disruption of the less massive WDduring the merger and the formation of a shell burning non-degenerate starincorporating the ONeMg core. During the shell burning stage a large fractionof the envelope is lost to the wind, while mass and angular momentum are addedto the core. As a result, the electron-capture collapse occurs with a smallenvelope mass, after $\sim 10^2-10^4$ years. During the formation of a neutronstar as little as $\sim 10^{-2} M_\odot $ of the material is ejected at thebounce-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 magneticfield is amplified. The ensuing fast magnetically-dominated relativistic windfrom the newly formed neutron star shocks against the ejecta, and later againstthe wind. The radiation-dominated forward shock produces the long-lastingoptical afterglow, while the termination shock of the relativistic windproduces 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 theafterglow spectrum. The model explains many of the puzzling properties ofFBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing abright optical peak, afterglow high energy emission of similar luminosity tothe optical, and late infra-red features.

FBOTs and AT2018cow following electron-capture collapse of merged white dwarfs