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Luminous Red Novae (LRNe) are astrophysical transients associated with thepartial ejection of a binary system's common envelope (CE) shortly before itsmerger. Here we present the results of our photometric and spectroscopicfollow-up campaign of AT2018bwo (DLT18x), a LRN discovered in NGC45, andinvestigate its progenitor system using binary stellar-evolution models. Thetransient reached a peak magnitude of $M_r=-10.97\pm0.11$ and maintained thisbrightness during its optical plateau of $t_p = 41\pm5$days. During this phase,it showed a rather stable photospheric temperature of ~3300K and a luminosityof ~$10^{40}$erg/s. The photosphere of AT2018bwo at early times appeared largerand cooler than other similar LRNe, likely due to an extended mass-loss episodebefore the merger. Towards the end of the plateau, optical spectra showed areddened continuum with strong molecular absorption bands. The reprocessedemission by the cooling dust was also detected in the mid-infrared bands ~1.5years after the outburst. Archival Spitzer and Hubble Space Telescope datataken 10-14 years before the transient event suggest a progenitor star with$T_{prog}\sim 6500$K, $R_{prog}\sim 100R_{\odot}$ and $L_{prog}\sim2\times10^4L_{\odot}$, and an upper limit for optically thin warm (1000 K) dustmass of $M_d<10^{-6}M_{\odot}$. Using stellar binary-evolution models, wedetermined the properties of binary systems consistent with the progenitorparameter 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-starevolution models. The system, consistent with a yellow-supergiant primary, waslikely 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.

The luminous red nova AT 2018bwo in NGC 45 and its binary yellow supergiant progenitor