  Mergers of neutron stars (NS+NS) or neutron stars and stellar mass black
holes (NS+BS) eject a small fraction of matter with a sub-relativistic
velocity. Upon rapid decompression nuclear density medium condenses into
neutron rich nuclei, most of them radioactive. Radioactivity provides a long
term heat source for the expanding envelope. A brief transient has the peak
luminosity in the supernova range, and the bulk of radiation in the UV --
Optical domain. We present a very crude model of the phenomenon, and simple
analytical formulae which may be used to estimate the parameters of a transient
as a function of poorly known input parameters.
  The mergers may be detected with high redshift supernova searches as rapid
transients, many of them far away from the parent galaxies. It is possible that
the mysterious optical transients detected by Schmidt et al. (1998) are related
to neutron star mergers as they typically have no visible host galaxy.
