The HARPS search for southern extra-solar planets XV. Six long-period giant planets around BD-17 0063, HD 20868, HD 73267, HD 131664, HD 145377, and HD 153950

We report the discovery of six new substellar companions of main-sequence stars, detected by multiple Doppler measurements with the instrument HARPS installed on the ESO 3.6 m telescope, La Silla, Chile. These extrasolar planets orbit the stars BD-17 0063, HD 20868, HD 73267, HD 131664, HD 145377, and HD 153950. The orbital characteristics that reproduce the observed data are presented, as well as the stellar and planetary parameters. Masses of the companions range from 2 to 18 Jupiter masses, and periods range from 100 to 2000 days. The observational data are carefully analysed for activity-induced effects, and we conclude that the observed radial velocity variations are of exoplanetary origin. Of particular interest is the very massive planet (or brown-dwarf companion) orbiting the metal-rich HD 131664 with m(2) sin i = 18.15 M-Jup and a 5.34-year orbital period. These new discoveries are consistent with the observed statistical properties of exoplanet samples known so far.