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Precise radial-velocity measurements for data acquired with the HARPSspectrograph infer that three planets orbit the M4 dwarf star GJ876. Inparticular, we confirm the existence of planet "d", which orbits every 1.93785days. We find that its orbit may have significant eccentricity (e=0.14), anddeduce a more accurate estimate of its minimum mass of 6.3 Earth masses.Dynamical modeling of the HARPS measurements combined with literaturevelocities from the Keck Observatory strongly constrain the orbitalinclinations of the "b" and "c" planets. We find that i_b = 48.9 degrees andi_c = 48.1 degrees, which infers the true planet masses of M_b = 2.64 Jupitermasses and M_c = 0.83 Jupiter masses, respectively. Radial velocities alone, inthis favorable case, can therefore fully determine the orbital architecture ofa multi-planet system, without the input from astrometry or transits.  The orbits of the two giant planets are nearly coplanar, and their 2:1 meanmotion resonance ensures stability over at least 5 Gyr. The libration amplitudeis smaller than 2 degrees, suggesting that it was damped by some dissipativeprocess during planet formation. The system has space for a stable fourthplanet in a 4:1 mean motion resonance with planet "b", with a period around 15days. The radial velocity measurements constrain the mass of this possibleadditional planet to be at most that of the Earth.

The HARPS search for southern extra-solar planets XIX. Characterization and dynamics of the GJ876 planetary system