  The detection of gravitational waves from coalescing binary neutron stars
represents another milestone in gravitational-wave astronomy. However, since
LIGO is currently not as sensitive to the merger/ringdown part of the waveform,
the possibility that such signals are produced by a black hole-neutron star
binary can not be easily ruled out without appealing to assumptions about the
underlying compact object populations. We review a few astrophysical channels
that might produce black holes below 3 $M_{\odot}$ (roughly the upper bound on
the maximum mass of a neutron star), as well as existing constraints for these
channels. We show that, due to the uncertainty in the neutron star equation of
state, it is difficult to distinguish gravitational waves from a binary neutron
star system, from those of a black hole-neutron star system with the same
component masses, assuming Advanced LIGO sensitivity. This degeneracy can be
broken by accumulating statistics from many events to better constrain the
equation of state, or by third-generation detectors with higher sensitivity to
the late spiral to post-merger signal. We also discuss the possible differences
in electromagnetic counterparts between binary neutron star and low mass black
hole-neutron star mergers, arguing that it will be challenging to definitively
distinguish the two without better understanding of the underlying
astrophysical processes.
