Binary black hole–neutron stars have received significantly less attention than binary black holes or binary neutron stars. No black hole–neutron star binary has been identified to date. However, stellar population synthesis models suggest that such systems represent a significant fraction of all compact binary mergers ultimately visible in gravitational waves by the LIGO detector. In addition, the study of black hole–neutron star mergers is important in light of the localizations of short-hard gamma-ray bursts. These GRB sources are found in galactic regions of low star-formation devoid of supernovae associations, ruling out massive stars as progenitors: massive stars have very short lifetimes and would need to be replenished more rapidly than is possible in low star-formation regions to account for these bursts. A more plausible progenitor for a short-hard GRB is a compact binary containing a neutron star, i.e., either a binary neutron star or binary black holeneutron star. The short-hard burst time scales and energetics are consistent with GRB models based on the coalescence of such compact binaries, and the evolution time scale of over 1 Gyr between formation and merger is consistent with the low star-formation rate.

Black hole-neutron star binaries can merge in two distinct ways. The neutron star may either be tidally disrupted by the black hole companion before being consumed, or it may be swallowed by the black hole more or less intact.