The HARPS search for southern extra-solar planets XXXVI. Planetary systems and stellar activity of the M dwarfs GJ 3293, GJ 3341, and GJ 3543

Context. Planetary companions of a fixed mass induce reflex motions with a larger amplitude around lower-mass stars, which adds to making M dwarfs excellent targets for extra-solar planet searches. The most recent velocimeters with a stability of similar to 1 m s(-1) can detect very low-mass planets out to the habitable zone of these stars. Low-mass small planets are abundant around M dwarfs, and most of the known potentially habitable planets orbit one of these cool stars. Aims. Our M-dwarf radial velocity monitoring with HARPS on the ESO 3.6 m telescope at La Silla observatory makes a major contribution to this sample. Methods. We present here dense radial velocity (RV) time series for three M dwarfs observed over similar to five years: GJ 3293 (0.42 M-circle dot), GJ 3341 (0.47 M-circle dot), and GJ 3543 (0.45 M-circle dot). We extracted these RVs through minimum (2)(X) -matching of each spectrum against a stack of all observed spectra for the same star that has a high signal-to-noise ratio. We then compared potential orbital signals against several stellar activity indicators to distinguish the Keplerian variations induced by planets from the spurious signals that result from rotational modulation of stellar surface inhomogeneities and from activity cycles. Results. Two Neptune-mass planets - m sin(i) = 1.4 +/- 0.1 and 1.3 +/- 0.1M(nept) - orbit GJ 3293 with periods P = 30.60 +/- 0.02 d and P = 123.98 +/- 0.38 d, possibly together with a super-Earth - m sin(i) similar to 7.9 +/- 1.4 M-circle plus - with period P = 48.14 +/- 0.12 d. A super-Earth - m sin(i) similar to 6.1 M-circle plus - orbits GJ 3341 with P = 14.207 +/- 0.007 d. The RV variations of GJ 3543, on the other hand, reflect its stellar activity rather than planetary signals.