A remarkable recurrent nova in M31 - The optical observations

Context: In late November 2013 a fifth eruption in five years of the M31 recurrent nova M31N 2008-12a was announced. Aims: In this Letter we address the optical lightcurve and progenitor system of M31N 2008-12a. Methods: Optical imaging data of the 2013 eruption from the Liverpool Telescope, La Palma, and Danish 1.54m Telescope, La Silla, and archival Hubble Space Telescope near-IR, optical and near-UV data are astrometrically and photometrically analysed. Results: Photometry of the 2013 eruption, combined with three previous eruptions, enabled construction of a template light curve of a very fast nova, t2(V)~4 days. The archival data allowed recovery of the progenitor system in optical and near-UV data, indicating a red-giant secondary with bright accretion disk, or alternatively a system with a sub-giant secondary but dominated by a disk. Conclusions: The eruptions of M31N 2008-12a, and a number of historic X-ray detections, indicate a unique system with a recurrence timescale of ~1 year. This implies the presence of a very high mass white dwarf and a high accretion rate. The recovered progenitor system is consistent with such an elevated rate of accretion.We encourage additional observations, especially towards the end of 2014.