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We present 0.9 - 2.5 um resolved spectra for the ultracool binary WISEPCJ121756.91+162640.2AB. The system consists of a pair of brown dwarfs thatstraddles the currently defined T/Y spectral type boundary. We use syntheticspectra generated by model atmospheres that include chloride and sulfide clouds(Morley et al.), the distance to the system (Dupuy & Kraus), and the radius ofeach component based on evolutionary models (Saumon & Marley) to determine aprobable range of physical properties for the binary. The effective temperatureof the T8.5 primary is 550 - 600 K, and that of the Y0 - Y0.5 secondary is 450K. The atmospheres of both components are either free of clouds or haveextremely thin cloud layers. We find that the masses of the primary andsecondary are 30 and 22 M_Jup, respectively, and that the age of the system is4 - 8 Gyr. This age is consistent with astrometric measurements (Dupuy & Kraus)that show that the system has kinematics intermediate between those of the thinand thick disks of the Galaxy. An older age is also consistent with anindication by the H - K colors that the system is slightly metal-poor.

Resolved Spectroscopy of the T8.5 and Y0-0.5 Binary WISEPC J121756.91+162640.2AB