  We present a numerical-relativity simulation of a black hole - neutron star
merger in scalar-tensor (ST) gravity with binary parameters consistent with the
gravitational wave event GW200115. In this exploratory simulation, we consider
the Damour-Esposito-Farese extension to Brans-Dicke theory, and maximize the
effect of spontaneous scalarization by choosing a soft equation of state and ST
theory parameters at the edge of known constraints. We extrapolate the
gravitational waves, including tensor and scalar (breathing) modes, to future
null-infinity. The numerical waveforms undergo ~ 22 wave cycles before the
merger, and are in good agreement with predictions from post-Newtonian theory
during the inspiral. We find the ST system evolves faster than its
general-relativity (GR) counterpart due to dipole radiation, merging a full
gravitational-wave cycle before the GR counterpart. This enables easy
differentiation between the ST waveforms and GR in the context of parameter
estimation. However, we find that dipole radiation's effect may be partially
degenerate with the NS tidal deformability during the late inspiral stage, and
a full Bayesian analysis is necessary to fully understand the degeneracies
between ST and binary parameters in GR.
