Limits to Tertiary Astrometric Companions in Binary Systems

The Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) has monitored 37 subarcsecond binary systems to determine whether their Keplerian orbits are perturbed by faint astrometric companions to either star. Software has been developed to evaluate the regions in a companion mass-period phase space in which the PHASES observations can exclude the possibility of face-on orbit perturbations. We present results for eight systems for which astrometric companions with masses as small as those of giant planets can be excluded.