Program: GN-2024B-Q-217

Red novae (RNe) are rare (with only 20 known to-date) transients arising from binaries undergoing common envelope evolution (CEE). They are thought to be the observable manifestation of the envelope ejection, likely followed by stellar merger. CEE is one of the most important and least-understood phases of binary evolution, affecting our understanding of, among others, those binary compact objects that are sources of gravitational waves. Studying properties of RNe and determining their rate provide an opportunity to constrain the common envelope phase and thereby advance our understanding of binary evolution. Each RN discovery is thus a big step forward. M31, which is routinely monitored by ongoing time-domain surveys, is an ideal target to discover RNe. Fainter RNe are known to be more common than their brighter counterparts, however, their luminosities overlap with those of ordinary slow novae, tens of which are discovered each year and not followed up spectroscopically. The faint RNe are thus hiding as ordinary novae in M31, and only spectroscopic monitoring until the late phase can distinguish these events, by uncovering their characteristic redward evolution with broad molecular absorption bands in their optical spectra. Here we propose to use Gemini to obtain the late-time optical spectra of two RN candidates in 2024B to confirm their RN nature.