  We report the observation of a coalescing compact binary with component
masses $2.5\text{-}4.5~M_\odot$ and $1.2\text{-}2.0~M_\odot$ (all measurements
quoted at the 90% credible level). The gravitational-wave signal
GW230529_181500 was observed during the fourth observing run of the
LIGO-Virgo-KAGRA detector network on 2023 May 29 by the LIGO Livingston
Observatory. The primary component of the source has a mass less than
$5~M_\odot$ at 99% credibility. We cannot definitively determine from
gravitational-wave data alone whether either component of the source is a
neutron star or a black hole. However, given existing estimates of the maximum
neutron star mass, we find the most probable interpretation of the source to be
the coalescence of a neutron star with a black hole that has a mass between the
most massive neutron stars and the least massive black holes observed in the
Galaxy. We provisionally estimate a merger rate density of
$55^{+127}_{-47}~\text{Gpc}^{-3}\,\text{yr}^{-1}$ for compact binary
coalescences with properties similar to the source of GW230529_181500; assuming
that the source is a neutron star-black hole merger, GW230529_181500-like
sources constitute about 60% of the total merger rate inferred for neutron
star-black hole coalescences. The discovery of this system implies an increase
in the expected rate of neutron star-black hole mergers with electromagnetic
counterparts and provides further evidence for compact objects existing within
the purported lower mass gap.
