  Four XMM-Newton observations of the core of M31, spaced at 6 month intervals,
show that the brightest point X-ray source, RX J0042.6+4115, has a 0.4-10 keV
luminosity of ~5 10^38 erg/s, and exhibits significant variability in intensity
and X-ray spectrum over a time scale of ~100 s including hard flares; such
behaviour is only observed in Z-sources and transient blackhole binaries in our
Galaxy. The lightcurves, X-ray spectra and hardness-intensity data from the
four XMM-Newton observations all strongly suggest that it is a Z-source,
bringing the total number of known Z-sources to nine.
