  Luminous Red Novae (LRNe) are astrophysical transients associated with the
partial ejection of a binary system's common envelope (CE) shortly before its
merger. Here we present the results of our photometric and spectroscopic
follow-up campaign of AT2018bwo (DLT18x), a LRN discovered in NGC45, and
investigate its progenitor system using binary stellar-evolution models. The
transient reached a peak magnitude of $M_r=-10.97\pm0.11$ and maintained this
brightness during its optical plateau of $t_p = 41\pm5$days. During this phase,
it showed a rather stable photospheric temperature of ~3300K and a luminosity
of ~$10^{40}$erg/s. The photosphere of AT2018bwo at early times appeared larger
and cooler than other similar LRNe, likely due to an extended mass-loss episode
before the merger. Towards the end of the plateau, optical spectra showed a
reddened continuum with strong molecular absorption bands. The reprocessed
emission by the cooling dust was also detected in the mid-infrared bands ~1.5
years after the outburst. Archival Spitzer and Hubble Space Telescope data
taken 10-14 years before the transient event suggest a progenitor star with
$T_{prog}\sim 6500$K, $R_{prog}\sim 100R_{\odot}$ and $L_{prog}\sim
2\times10^4L_{\odot}$, and an upper limit for optically thin warm (1000 K) dust
mass of $M_d<10^{-6}M_{\odot}$. Using stellar binary-evolution models, we
determined the properties of binary systems consistent with the progenitor
parameter space. For AT2018bwo, we infer a primary mass of 12-16 $M_{\odot}$,
which is 9-45% larger than the ~11$M_{\odot}$ obtained using single-star
evolution models. The system, consistent with a yellow-supergiant primary, was
likely in a stable mass-transfer regime with -2.4<log
($\dot{M}/M_{\odot}$/yr)<-1.2 a decade before the main instability occurred.
During the dynamical merger, the system would have ejected 0.15-0.5$M_{\odot}$
with a velocity of ~500 km/s.
