  {"Bare collapse", the collapse of a bare stellar core to a neutron star with
a very small mass ejection links two seemingly unrelated phenomena: the
formation of binary neutron star (BNS) systems and the observations of fast and
luminous optical transients. We carried out calculations of the collapse due to
electron-capture of both evolutionary and synthetic isentropic bare stellar
cores. We find that the collapse results in {the formation of} a light ~ 1.3
solar mass neutron star and {an} ejection of ~0.1 solar mass at ~0.1c. The
outer shell of the ejecta is composed of Ni56 that can power an ultra-stripped
supernova. The models we explored can explain most of the observed fast optical
flares but not the brightest ones. Collapse of cores surrounded by somewhat
more massive envelopes can produce larger amounts of Ni56 and explain brighter
flares. Alternatively, those events can arise due to interaction of the very
energetic ejecta with winds that were ejected from the progenitor a few days
before the collapse.
