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We present observations of M31LRN 2015 (MASTER OT J004207.99+405501.1),discovered in M31 in 2015 January, and identified as a rare and enigmaticluminous red nova (LRN). Spectroscopic and photometric observations obtained bythe Liverpool Telescope showed the LRN becoming extremely red as it faded fromits M(V) = -9.4 +/- 0.2 peak. Early spectra showed strong Halpha emission thatweakened over time as a number of absorption features appeared, including Na ID and Ba II. At later times strong TiO absorption bands were also seen. Asearch of archival Hubble Space Telescope data revealed a luminous red sourceto be the likely progenitor system, with pre-outburst Halpha emission alsodetected in ground-based data. The outburst of M31LRN 2015 shows manysimilarities, both spectroscopically and photometrically, with that of V838Mon, the best studied LRN. We finally discuss the possible progenitorscenarios.

A Luminous Red Nova in M31 and its Progenitor System