Vol. 582
In section 1. Letters

A remarkable recurrent nova in M 31: The 2010 eruption recovered and evidence of a six-month period

Recurrent novae are cataclysmic variables that display brief, but luminous eruptions. They are the result of a thermonuclear runaway occurring on the surface of the white dwarf, which is accreting matter at a variable rate from a non-degenerate companion. As such, these systems are potentially viable candidates for type Ia supernova progenitors. Furthermore, their brightness means that they are amenable to being discovered in nearby galaxies. M31N 2008-12a, in the Andromeda galaxy, is particularly remarkable because it has been been caught in outburst no fewer than nine times, with six of these occurring between 2008 and 2014. Somewhat puzzlingly is that no outburst was reported in 2010. Based on a trawl through archival data, the authors of the above study were able to convincingly recover a source that was spatially coincident with the 2012 outburst. The recovery of the 2010 eruption implies a recurrence timescale of about half a year, making M31N 2008-12a the most rapidly recurring system known to date. More importantly, the authors were able to roughly predict the epoch of the next eruption. Remarkably, while the Letter was being refereed, the 2015 eruption was reported, somewhat ahead of the prediction, but still consistent with expectations.