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Context: How planet properties depend on stellar mass is a key diagnostic ofplanetary formation mechanisms. Aims: This motivates planet searches aroundstars which are significantly more massive or less massive than the Sun, and inparticular our radial velocity search for planets around very-low mass stars.Methods: As part of that program, we obtained measurements of GJ 674, an M2.5dwarf at d=4.5 pc, which have a dispersion much in excess of their internalerrors. An intensive observing campaign demonstrates that the excess dispersionis due to two superimposed coherent signals, with periods of 4.69 and 35 days.Results: These data are well described by a 2-planet Keplerian model where eachplanet has a ~11 Mearth minimum mass. A careful analysis of the (low level)magnetic activity of GJ 674 however demonstrates that the 35-day periodcoincides with the stellar rotation period. This signal therefore originates ina spot inhomogeneity modulated by stellar rotation. The 4.69-day signal on theother hand is caused by a bona-fide planet, GJ 674b. Conclusion: Its detectionadds to the growing number of Neptune-mass planets around M-dwarfs, andreinforces the emerging conclusion that this mass domain is much more populatedthan the jovian mass range. We discuss the metallicity distributions of M dwarfwith and without planets and find a low 11% probability that they are drawnfrom the same parent distribution. Moreover, we find tentative evidence thatthe host star metallicity correlates with the total mass of their planetarysystem.

The HARPS search for southern extra-solar planets. X. A m sin i = 11 Mearth planet around the nearby spotted M dwarf GJ 674