A second giant planet in 3:2 mean-motion resonance in the HD 204313 system

We present eight years of high-precision radial velocity (RV) data for HD 204313 from the 2.7m Harlan J. Smith Telescope at McDonald Observatory. The star is known to have a giant planet (Msin i = 3.5 M J ) on a 1900 day orbit, and a Neptune-mass planet at 0.2AU. Using our own data in combination with the published CORALIE RVs of Segransan et al., we discover an outer Jovian (Msin i = 1.6 M J ) planet with P 2800 days. Our orbital fit suggests that the planets are in a 3:2 mean motion resonance, which would potentially affect their stability. We perform a detailed stability analysis and verify that the planets must be in resonance