LP 876-10 is a nearby active M4 dwarf in Aquarius at a distance of 7.6 pc .
The star is a new addition to the 10-pc census , with a parallax measured via the Research Consortium on Nearby Stars ( RECONS ) astrometric survey on the Small & Moderate Aperture Research Telescope System ’ s ( SMARTS ) 0.9-m telescope .
We demonstrate that the astrometry , radial velocity , and photometric data for LP 876-10 are consistent with the star being a third , bound , stellar component to the Fomalhaut multiple system , despite the star lying nearly 6 ^ { \circ } away from Fomalhaut A in the sky .
The 3D separation of LP 876-10 from Fomalhaut is only 0.77 \pm 0.01 pc , and 0.987 \pm 0.006 pc from TW PsA ( Fomalhaut B ) , well within the estimated tidal radius of the Fomalhaut system ( 1.9 pc ) .
LP 876-10 shares the motion of Fomalhaut within \sim 1 km/s , and we estimate an interloper probability of \sim 10 ^ { -5 } .
Neither our echelle spectroscopy nor astrometry are able to confirm the close companion to LP 876-10 reported in the Washington Double Star Catalog ( WSI 138 ) .
We argue that the Castor Moving Group to which the Fomalhaut system purportedly belongs , is likely to be a dynamical stream , and hence membership to the group does not provide useful age constraints for group members .
LP 876-10 ( Fomalhaut C ) has now risen from obscurity to become a rare example of a field M dwarf with well-constrained age ( 440 \pm 40 Myr ) and metallicity .
Besides harboring a debris disk system and candidate planet , Fomalhaut now has two of the widest known stellar companions .