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We present the first X-ray and UV/optical observations of a very bright andfast nova in the disc of M31, M31N 2013-01b. The nova reached a peak magnitude$R\sim$15 mag and decayed by 2 magnitudes in only 3 days, making it one of thebrightest and fastest novae ever detected in Andromeda. From archivalmulti-band data we have been able to trace its fast evolution down to $U>21$mag in less than two weeks and to uncover for the first time the Super-SoftX-ray phase, whose onset occurred 10-30 days from the optical maximum. TheX-ray spectrum is consistent with a blackbody with a temperature of $\sim$50 eVand emitting radius of $\sim$4$\times 10^{9}$ cm, larger than a white dwarfradius, indicating an expanded region. Its peak X-ray luminosity, 3.5$\times10^{37}$ erg s$^{-1}$, locates M31N 2013-01b among the most luminous novae inM31. We also unambiguously detect a short 1.28$\pm$0.02 h X-ray periodicitythat we ascribe to the binary orbital period, possibly due to partial eclipses.This makes M31N 2013-01b the first nova in M31 with an orbital perioddetermined. The short period also makes this nova one of the few known belowthe 2-3 h orbital period gap. All the observed characteristics stronglyindicate that M31N 2013-01b harbours a massive white dwarf and a very low-masscompanion, consistent with being a nova belonging to the disc population of theAndromeda Galaxy.

The first orbital period of a very bright and fast Nova in M31: M31N 2013-01b