Differential astrometry measurements from the Palomar High-precision Astrometric Search for Exoplanet Systems ( PHASES ) are used to constrain the astrometric orbit of the previously known \lesssim 2 day subsystem in the triple system 63 Gem A and have detected a previously unknown 2 year Keplerian wobble superimposed on the visual orbit of the much longer period ( 213 years ) binary system HR 2896 .
63 Gem A was already known to be triple from spectroscopic work , and absorption lines from all 3 stars can be identified and their individual Doppler shifts measured ; new velocities for all three components are presented to aid in constraining the orbit and measuring the stellar masses .
In fact , 63 Gem itself is a sextuple system : the hierarchical triple ( Aa1-Aa2 ) -Ab ( in which Aa1 and Aa2 orbit each other with a rapid period just under 2 days , and Ab orbits these every 2 years ) , plus three distant common proper motion companions .
The very small astrometric perturbation caused by the inner pair in 63 Gem A stretches the limits of current astrometric capabilities , but PHASES observations are able to constrain the orientation of the orbit .
The two bright stars comprising the HR 2896 long period ( 213 year ) system have a combined spectral type of K0III and the newly detected object ’ s mass estimate places it in the regime of being a M dwarf .
The motion of the stars are slow enough that their spectral features are always blended , preventing Doppler studies .
The PHASES measurements and radial velocities ( when available ) have been combined with lower precision single-aperture measurements covering a much longer timeframe ( from eyepiece measurements , speckle interferometry , and adaptive optics ) to improve the characterization of the long period orbits in both binaries .
The visual orbits of the short and long period systems are presented for both systems , and used to calculate two possible values of the mutual inclinations between inner and outer orbits of 152 \pm 12 degrees or a less likely value of 31 \pm 11 degrees for 63 Gem A and 10.2 \pm 2.4 degrees or 171.2 \pm 2.8 degrees for HR 2896 .
The first is not coplanar , whereas the second is either nearly coplanar or anti-coplanar .