HD 202206: A Circumbinary Brown Dwarf System

Using Hubble Space Telescope Fine Guidance Sensor astrometry and previously published radial velocity measures, we explore the exoplanetary system HD 202206. Our modeling results in a parallax, pi(abs) = 21.96 +/- 0.12 milliseconds of arc, a mass for HD 202206 B of M-B =0.089(-0.006)(+0.007) M-circle dot, and a mass for HD 202206 c of M-c = 17.9(-1.8)(+2.9) M-Jup. HD 202206 is a nearly face-on G + M binary orbited by a brown dwarf. The system architecture that we determine supports past assertions that stability requires a 5: 1 mean motion resonance (we find a period ratio, P-c/P-B= 4.92 +/- 0.04) and coplanarity (we find a mutual inclination, Phi = 6 degrees +/- 2 degrees).