  2013 FY27 is the ninth intrinsically brightest Trans-Neptunian Object (TNO).
We observed 2013 FY27 at thermal wavelengths with ALMA and in the optical with
Magellan to determine its size and albedo for the first time and compare it to
other dwarf planets. The geometric albedo of 2013 FY27 was found to be 0.17
-0.030+0.045, giving an effective diameter of D = 765 +80-85 km. 2013 FY27 has
a size within the transition region between the largest few TNOs that have
higher albedos and higher densities than smaller TNOs. No significant
short-term optical light curve was found, with variations less than 0.06 mags
over hours and days. The Sloan optical colors of 2013 FY27 are g-r=0.76 +-0.02
and r-i=0.31 +-0.03 mags, which is a moderately red color. This color is
different than the neutral or ultra-red colors found for the ten largest TNOs,
making 2013 FY27 one of the largest known moderately red TNOs, which only start
to be seen, and in abundance, at diameters less than 800 km. This suggests
something physically different might be associated with TNOs larger than 800
km. It could be that moderately red surfaces are older or less ice rich and
TNOs larger than 800 km have fresher surfaces or are able to hold onto more
volatile ices. Its also possible TNOs larger than 800 km are more fully
differentiated, giving them different surface compositions. A satellite at 0.17
arcsec away and 3.0 +-0.2 mags fainter than 2013 FY27 was found through Hubble
Space Telescope observations. Almost all the largest TNOs have satellites, and
the relative small size of 2013 FY27's satellite suggests it was created
through a direct collision, similar to satellites known around the largest
TNOs. Assuming the satellite has a similar albedo as the primary, it is about
190 km in diameter, making the primary D = 740 +85-90 km.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics;Solar and Stellar Astrophysics