16725

On the cusp of the JWST era comma we are still lacking mass measurements for the coldest brown dwarfs comma even though mass is crucial for testing substellar models. We propose a 3-year orbit monitoring program to obtain the first sample of dynamical masses at temperatures of 400-800 K. With projected separations of only 1-3 AU comma our targets are among the tightest and faintest substellar visual binaries ever found comma discovered at the extreme limits of existing facilities. Resolved astrometry comma combined with existing parallax data comma is needed to measure their orbits comma and only HST can deliver the high-precision monitoring required. These systems comma composed entirely of T8-Y0 dwarfs comma are likely to be the only such binaries amenable to direct mass measurements within the decade.