We report the discovery of a T8.5 dwarf , which is a companion to the M4 dwarf Wolf 940 .
At a distance of 12.50 ^ { +0.75 } _ { -0.67 } pc , the angular separation of 32″ corresponds to a projected separation of 400 AU .
The M4 primary displays no H \alpha emission , and we apply the age-activity relations of West et al .
to place a lower limit on the age of the system of 3.5 Gyr .
Weak H \alpha absorption suggests some residual activity and we estimate an upper age limit of 6 Gyr .
We apply the relations of Bonfils et al for V - K _ { s } and M _ { K _ { s } } to determine the metallicity , { [ Fe / H ] } = -0.06 \pm 0.20 for Wolf 940A , and by extension the T8.5 secondary , Wolf 940B .
We have obtained JHK NIRI spectroscopy and JHKL ^ { \prime } photometry of Wolf 940B , and use these data , in combination with theoretical extensions , to determine its bolometric flux , F _ { bol } = 1.75 \pm 0.18 \times 10 ^ { -16 } { W } m ^ { -2 } and thus its luminosity \log ( L _ { * } / \mbox { $L _ { \odot } $ } ) = -6.07 \pm 0.04 .
Using the age constraints for the system , and evolutionary structural models of Baraffe et al .
we determine T _ { eff } = 570 \pm 25 K and \log g = 4.75 - 5.00 for Wolf 940B , based on its bolometric luminosity .
This represents the first determination of these properties for a T8+ dwarf that does not rely on the fitting of T-dwarf spectral models .
This object represents the first system containing a T8+ dwarf for which fiducial constraints on its properties are available , and we compare its spectra with those of the latest very cool BT-Settl models .
This clearly demonstrates that the use of the ( W _ { J } , K / J ) spectral ratios ( used previously to constrain T _ { eff } and \log g ) would have over-estimated T _ { eff } by \sim 100 K .