  An ultra-stripped supernova (SN) is an explosion of a helium or C+O star
whose outer envelope has been stripped away by a companion neutron star. A
double neutron star (DNS) binary is believed to be left after the explosion,
which will emit the gravitational wave later at the coalescence. Recent
detections of a few candidates for the ultra-stripped SN have constrained the
properties of the explosion and the progenitor, but little information is given
as to whether the remnant DNS binary will merge within the cosmic age. A large
fraction of the material stripped away from the helium star through the binary
interaction is expected to escape from the system and form circumstellar
material (CSM). The CSM should be traced by radio emission induced by the
collision with the SN ejecta. Based on the stellar evolution models previously
developed, we calculate the expected radio luminosities from ultra-stripped
SNe. We find that high radio luminosity at its maximum can be an indicator of
small separation of a DNS binary leading to its merger within the cosmic age.
Our results can be used to optimize the strategy for the radio follow-up
observations such as observational epochs and frequencies.
