  We report the observation of gravitational waves from two compact binary
coalescences in LIGO's and Virgo's third observing run with properties
consistent with neutron star-black hole (NSBH) binaries. The two events are
named GW200105_162426 and GW200115_042309, abbreviated as GW200105 and
GW200115; the first was observed by LIGO Livingston and Virgo, and the second
by all three LIGO-Virgo detectors. The source of GW200105 has component masses
$8.9^{+1.2}_{-1.5}\,M_\odot$ and $1.9^{+0.3}_{-0.2}\,M_\odot$, whereas the
source of GW200115 has component masses $5.7^{+1.8}_{-2.1}\,M_\odot$ and
$1.5^{+0.7}_{-0.3}\,M_\odot$ (all measurements quoted at the 90% credible
level). The probability that the secondary's mass is below the maximal mass of
a neutron star is 89%-96% and 87%-98%, respectively, for GW200105 and GW200115,
with the ranges arising from different astrophysical assumptions. The source
luminosity distances are $280^{+110}_{-110}$ Mpc and $300^{+150}_{-100}$ Mpc,
respectively. The magnitude of the primary spin of GW200105 is less than 0.23
at the 90% credible level, and its orientation is unconstrained. For GW200115,
the primary spin has a negative spin projection onto the orbital angular
momentum at 88% probability. We are unable to constrain spin or tidal
deformation of the secondary component for either event. We infer a NSBH merger
rate density of $45^{+75}_{-33}\,\mathrm{Gpc}^{-3} \mathrm{yr}^{-1}$ when
assuming GW200105 and GW200115 are representative of the NSBH population, or
$130^{+112}_{-69}\,\mathrm{Gpc}^{-3} \mathrm{yr}^{-1}$ under the assumption of
a broader distribution of component masses.
