* Astronomy

Title: A remarkable recurrent nova in M31 - The optical observations
Author: M. J. Darnley (1), S. C. Williams (1), M. F. Bode (1), M. Henze (2), J.-U. Ness (2), A. W. Shafter (3), K. Hornoch (4), V. Votruba (4) ((1) Astrophysics Research Institute, Liverpool John Moores University, UK, (2) European Space Astronomy Centre, Madrid, Spain, (3) Department of Astronomy, San Diego State University, US, (4) Astronomical Institute, Academy of Sciences, Ondrejov, Czech Republic)

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.

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