  We investigate light-curve and spectral properties of ultra-stripped
core-collapse supernovae. Ultra-stripped supernovae are the explosions of
heavily stripped massive stars which lost their envelopes via binary
interactions with a compact companion star. They eject only ~ 0.1 Msun and may
be the main way to form double neutron-star systems which eventually merge
emitting strong gravitational waves. We follow the evolution of an
ultra-stripped supernova progenitor until iron core collapse and perform
explosive nucleosynthesis calculations. We then synthesize light curves and
spectra of ultra-stripped supernovae using the nucleosynthesis results and
present their expected properties. Ultra-stripped supernovae synthesize ~ 0.01
Msun of radioactive 56Ni, and their typical peak luminosity is around 1e42
erg/s or -16 mag. Their typical rise time is 5 - 10 days. Comparing synthesized
and observed spectra, we find that SN 2005ek, some of the so-called
calcium-rich gap transients, and SN 2010X may be related to ultra-stripped
supernovae. If these supernovae are actually ultra-stripped supernovae, their
event rate is expected to be about 1 per cent of core-collapse supernovae.
Comparing the double neutron-star merger rate obtained by future
gravitational-wave observations and the ultra-stripped supernova rate obtained
by optical transient surveys identified with our synthesized light-curve and
spectral models, we will be able to judge whether ultra-stripped supernovae are
actually a major contributor to the binary neutron star population and provide
constraints on binary stellar evolution.
