We present 8 years of high-precision radial velocity (RV) data for HD 204313 from the 2.7 m Harlan J. Smith Telescope at McDonald Observatory. The star is known to have a giant planet (M sin i = 3.5 M_J) on a ~1900-day orbit, and a Neptune-mass planet at 0.2 AU. Using our own data in combination with the published CORALIE RVs of Segransan et al. (2010), we discover an outer Jovian (M sin i = 1.6 M_J) planet with P ~ 2800 days. Our orbital fit suggests the planets are in a 3:2 mean motion resonance, which would potentially affect their stability. We perform a detailed stability analysis, and verify the planets must be in resonance.
@article{arxiv.1205.3689,
title = {A Second Giant Planet in 3:2 Mean-Motion Resonance in the HD 204313 System},
author = {Paul Robertson and J. Horner and Robert A. Wittenmyer and Michael Endl and William D. Cochran and Phillip J. MacQueen and Erik J. Brugamyer and Attila E. Simon and Stuart I. Barnes and Caroline Caldwell},
journal= {arXiv preprint arXiv:1205.3689},
year = {2015}
}