  Chemically peculiar stars in eclipsing binary systems are rare objects that
allow the derivation of fundamental stellar parameters and important
information on the evolutionary status and the origin of the observed chemical
peculiarities. Here we present an investigation of the known eclipsing binary
system BD+09 1467 = V680 Mon. Using spectra from the Large Sky Area
Multi-Object Fiber Spectroscopic Telescope (LAMOST) and own observations, we
identify the primary component of the system as a mercury-manganese (HgMn/CP3)
star (spectral type kB9 hB8 HeB9 V HgMn). Furthermore, photometric time series
data from the Transiting Exoplanet Survey Satellite (TESS) indicate that the
system is a "heartbeat star", a rare class of eccentric binary stars with
short-period orbits that exhibit a characteristic signature near the time of
periastron in their light curves due to the tidal distortion of the components.
Using all available photometric observations, we present an updated ephemeris
and binary system parameters as derived from modelling of the system with the
ELISa code, which indicates that the secondary star has an effective
temperature of Teff = 8300+-200 K (spectral type of about A4). V680 Mon is only
the fifth known eclipsing CP3 star and the first one in a heartbeat binary.
Furthermore, our results indicate that the star is located on the zero-age main
sequence and a possible member of the open cluster NGC 2264. As such, it lends
itself perfectly for detailed studies and may turn out to be a keystone in the
understanding of the development of CP3 star peculiarities.
