We present Keck II laser guide star adaptive optics observations of the brown dwarf WISEP J061135.13 - 041024.0Â showing it is a binary with a component separation of 0 \farcs 4 .
This system is one of the six known resolved binaries in which the magnitude differences between the components show a reversal in sign between the Y / J band and the H / K bands .
Deconvolution of the composite spectrum results in a best fit binary solution with L9 and T1.5 components .
We also present a preliminary parallax placing the system at a distance of 21.2 \pm 1.3Â pc .
Using the distance and resolved magnitudes we are able to place WISEP J061135.13 - 041024.0AB on a color-absolute magnitude diagram , showing that this system contributes to the well-known “ J -band bump ” and the components ’ properties appear similar to other late-type L and early-type T dwarfs .
Fitting our data to a set of cloudy atmosphere models suggests the system has an age > 1 Gyr with WISE 0611 - 0410A having an effective temperature ( T _ { eff } ) of 1275-1325 K and mass of 64–65 M _ { Jup } , and WISE 0611 - 0410B having T _ { eff } = 1075–1115 K and mass 40–65 M _ { Jup } .