A remarkable recurrent nova in M 31: The optical observations

Context. In late Nov. 2013 the fifth eruption in five years of the M 31 recurrent nova M 31N 2008-12a was announced. Aims. In this Letter we address the optical lightcurve and progenitor system of M 31N 2008-12a. Methods. Optical imaging data of the 2013 eruption from the Liverpool Telescope, La Palma; Danish 1.54 m 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 lightcurve of a very fast nova (t(2) (V) similar or equal to 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 M 31N 2008-12a, and a number of historic X-ray detections, indicate a unique system with a recurrence timescale of similar to 1 yr. 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.