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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 keVluminosity of ~5 10^38 erg/s, and exhibits significant variability in intensityand X-ray spectrum over a time scale of ~100 s including hard flares; suchbehaviour is only observed in Z-sources and transient blackhole binaries in ourGalaxy. The lightcurves, X-ray spectra and hardness-intensity data from thefour XMM-Newton observations all strongly suggest that it is a Z-source,bringing the total number of known Z-sources to nine.Comment: Accepted for publication in A&A, 6 pages, 5 figure

Tracing a Z-track in the M 31 X-ray binary RX J0042.6+4115