The fourth United States Naval Observatory (USNO) CCD Astrograph Catalog, UCAC4 was released in August 2012 (double-sided DVD and CDS data center Vizier catalog I/322). It is the final release in this series and contains over 113 million objects; over 105 million of them with proper motions. UCAC4 is an updated version of UCAC3 with about the same number of stars also covering all-sky. Bugs were fixed, Schmidt plate survey data were avoided, and precise 5-band photometry were added for about half the stars. Astrograph observations have been supplemented for bright stars by FK6, Hipparcos and Tycho-2 data to compile a UCAC4 star catalog complete from the brightest stars to about magnitude R = 16. Epoch 1998 to 2004 positions are obtained from observations with the 20 cm aperture USNO Astrograph's "red lens", equipped with a 4k by 4k CCD. Mean positions and proper motions are derived by combining these observations with over 140 ground-and space-based catalogs, including Hipparcos/Tycho and the AC2000.2, as well as unpublished measures of over 5000 plates from other astrographs. For most of the faint stars in the Southern Hemisphere the first epoch plates from the Southern Proper Motion (SPM) program form the basis for proper motions, while the Northern Proper Motion (NPM) 1st epoch plates serve the same purpose for the rest of the sky. These data are supplemented by 2MASS near-IR photometry for about 110 million stars and 5-band (B,V,g,r,i) APASS data for over 51 million stars. Thus the published UCAC4, as were UCAC3 and UCAC2, is a compiled catalog with the UCAC observational program being a major component. The positional accuracy of stars in UCAC4 at mean epoch is about 15 to 100 mas per coordinate, depending on magnitude, while the formal errors in proper motions range from about 1 to 10 mas/yr depending on magnitude and observing history. Systematic errors in proper motions are estimated to be about 1 to 4 mas/yr.

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New astrometric reductions of the US Naval Observatory CCD Astrograph Catalog (UCAC) all-sky observations were performed from first principles using the TGAS stars in the 8 to 11 magnitude range as reference star catalog. Significant improvements in the astrometric solutions were obtained and the UCAC5 catalog of mean positions at a mean epoch near 2001 was generated. By combining UCAC5 with Gaia DR1 data new proper motions on the Gaia coordinate system for over 107 million stars were obtained with typical accuracies of 1 to 2 mas/yr (R = 11 to 15 mag), and about 5 mas/yr at 16th mag. Proper motions of most TGAS stars are improved over their Gaia data and the precision level of TGAS proper motions is extended to many millions more, fainter stars. External comparisons were made using stellar cluster fields and extragalactic sources. The TGAS data allow us to derive the limiting precision of the UCAC x, y data, which is significantly better than 1/100 pixel.

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 • away from Fomalhaut A in the sky. The 3D separation of LP 876-10 from Fomalhaut is only 0.77 ± 0.01 pc, and 0.987 ± 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 ∼1 km/s, and we estimate an interloper probability of ∼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 ± 40 Myr) and metallicity. Besides harboring a debris disk system and candidate planet, Fomalhaut now has two of the widest known stellar companions. Subject headings: binaries: visual -Stars: activity -Stars: fundamental parameters -Stars: individual (LP 876-10, Fomalhaut, TW PsA) -Stars: rotation sured. θ is the projected angular separation from Fomalhaut, and PA is the position angle as measured north through east. this paper, using positions from van Leeuwen (2007) and Roeser et al. (2010). In the Table and throughout the paper, Galactic velocities U and positions X are defined towards the Galactic center, V and Y are in the direction of Galactic rotation, and W and Z is towards the North Galactic pole.