We have discovered that SDSS J105213.51+442255.7 ( T 0.5 \pm 1.0 ) is a binary in Keck laser guide star adaptive optics imaging , displaying a large J -to- K -band flux reversal ( \Delta { J } = -0.45 \pm 0.09 mag , \Delta { K } = 0.52 \pm 0.05 mag ) .
We determine a total dynamical mass from Keck orbital monitoring ( 88 \pm 5 M _ { Jup } ) and a mass ratio by measuring the photocenter orbit from CFHT/WIRCam absolute astrometry ( M _ { B } / M _ { A } = 0.78 \pm 0.07 ) .
Combining these provides the first individual dynamical masses for any field L or T dwarfs , 49 \pm 3 M _ { Jup } for the L 6.5 \pm 1.5 primary and 39 \pm 3 M _ { Jup } for the T 1.5 \pm 1.0 secondary .
Such a low mass ratio for a nearly equal luminosity binary implies a shallow mass–luminosity relation over the L/T transition ( \Delta \log \mbox { $L _ { bol } $ } / \Delta \log { M } = 0.6 ^ { +0.6 } _ { -0.8 } ) .
This provides the first observational support that cloud dispersal plays a significant role in the luminosity evolution of substellar objects .
Fully cloudy models fail our coevality test for this binary , giving ages for the two components that disagree by 0.2 dex ( 2.0 \sigma ) .
In contrast , our observed masses and luminosities can be reproduced at a single age by ” hybrid ” evolutionary tracks where a smooth change from a cloudy to cloudless photosphere around 1300 K causes slowing of luminosity evolution .
Remarkably , such models also match our observed JHK flux ratios and colors well .
Overall , it seems that the distinguishing features SDSS J1052+4422AB , like a J -band flux reversal and high-amplitude variability , are normal for a field L/T binary caught during the process of cloud dispersal , given that the age ( 1.11 ^ { +0.17 } _ { -0.20 } Gyr ) and surface gravity ( \mbox { $ \log { g } $ } = 5.0 –5.2 ) of SDSS J1052+4422AB are typical for field ultracool dwarfs .