  The purpose of this study is to find transient X-ray sources in M31, and to
investigate and classify their nature. Three X-ray transients were observed
with Swift. For each of the three X-ray transients we use the Swift X-ray and
optical data together with observations from XMM-Newton and Chandra to
investigate the lightcurves and the spectra of the outburst, and thereby to
identify the source types. The outburst of XMMU J004215.8+411924 lasted for
about one month. The source had a hard power-law spectrum with a photon index
of 1.6. It was previously identified as a Be/X-ray binary based on the optical
identification with a star. However, we show that with improved source
coordinates it is clear that the optical source is not the counterpart to the
X-ray source. The source SWIFT J004217.3+411532 had a bright outburst, after
which it slowly decayed over half a year. The spectrum was soft, corresponding
to a thermal accretion disk with innermost temperature of 250-600 eV. The
source was not seen in the optical, and the soft spectrum indicates that the
source is most likely a black hole low mass X-ray binary. M31N 2006-11a is a
nova that was previously observed in the optical. We detected it both in X-rays
and UV with Swift half a year after the optical maximum, after which it decayed
below the Swift detection threshold within a month. The spectrum of the X-ray
transient can be modelled by a black-body with a temperature of 50 eV. We use
catalogues of X-ray transients in M31 to estimate their rate, and we find a
lower limit of 9/yr.
