We report the discovery of a widely separated ( 258 \farcs 3 \pm 0 \farcs 4 ) T dwarf companion to the Gl 570ABC system .
This new component , Gl 570D , was initially identified from the Two Micron All Sky Survey ( 2MASS ) .
Its near-infrared spectrum shows the 1.6 and 2.2 \micron CH _ { 4 } absorption bands characteristic of T dwarfs , while its common proper motion with the Gl 570ABC system confirms companionship .
Gl 570D ( M _ { J } = 16.47 \pm 0.07 ) is nearly a full magnitude dimmer than the only other known T dwarf companion , Gl 229B , and estimates of L = ( 2.8 \pm 0.3 ) x10 ^ { -6 } L _ { \sun } and T _ { eff } = 750 \pm 50 K make it significantly cooler and less luminous than any other known brown dwarf companion .
Using evolutionary models by Burrows et al .
and an adopted age of 2-10 Gyr , we derive a mass estimate of 50 \pm 20 M _ { Jup } for this object .