  We present the first X-ray and UV/optical observations of a very bright and
fast 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 the
brightest and fastest novae ever detected in Andromeda. From archival
multi-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-Soft
X-ray phase, whose onset occurred 10-30 days from the optical maximum. The
X-ray spectrum is consistent with a blackbody with a temperature of $\sim$50 eV
and emitting radius of $\sim$4$\times 10^{9}$ cm, larger than a white dwarf
radius, indicating an expanded region. Its peak X-ray luminosity, 3.5$\times
10^{37}$ erg s$^{-1}$, locates M31N 2013-01b among the most luminous novae in
M31. We also unambiguously detect a short 1.28$\pm$0.02 h X-ray periodicity
that 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 period
determined. The short period also makes this nova one of the few known below
the 2-3 h orbital period gap. All the observed characteristics strongly
indicate that M31N 2013-01b harbours a massive white dwarf and a very low-mass
companion, consistent with being a nova belonging to the disc population of the
Andromeda Galaxy.
