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We have discovered that SDSSJ105213.51+442255.7 (T0.5$\pm$1.0) is a binary inKeck laser guide star adaptive optics imaging, displaying a large J-to-K-bandflux reversal ($\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_{\rm Jup}$) and a mass ratio by measuring the photocenter orbit fromCFHT/WIRCam absolute astrometry ($M_B/M_A$ = 0.78$\pm$0.07). Combining theseprovides the first individual dynamical masses for any field L or T dwarfs,49$\pm$3 $M_{\rm Jup}$ for the L6.5$\pm$1.5 primary and 39$\pm$3 $M_{\rm Jup}$for the T1.5$\pm$1.0 secondary. Such a low mass ratio for a nearly equalluminosity binary implies a shallow mass$-$luminosity relation over the L/Ttransition ($\Delta$log$L_{\rm bol}$/$\Delta$log$M = 0.6^{+0.6}_{-0.8}$). Thisprovides the first observational support that cloud dispersal plays asignificant role in the luminosity evolution of substellar objects. Fullycloudy models fail our coevality test for this binary, giving ages for the twocomponents that disagree by 0.2 dex (2.0$\sigma$). In contrast, our observedmasses and luminosities can be reproduced at a single age by "hybrid"evolutionary tracks where a smooth change from a cloudy to cloudlessphotosphere around 1300 K causes slowing of luminosity evolution. Remarkably,such models also match our observed JHK flux ratios and colors well. Overall,it seems that the distinguishing features SDSSJ1052+4422AB, like a J-band fluxreversal and high-amplitude variability, are normal for a field L/T binarycaught during the process of cloud dispersal, given that the age(1.11$^{+0.17}_{-0.20}$ Gyr) and surface gravity (log$g$ = 5.0$-$5.2) ofSDSSJ1052+4422AB are typical for field ultracool dwarfs.

The Mass-Luminosity Relation in the L/T Transition: Individual Dynamical Masses for the New J-Band Flux Reversal Binary SDSSJ105213.51+442255.7AB