Calibrating cosmological chronometers: White dwarf masses via astrometry

In an effort to increase the number of accurate dynamical masses for white dwarfs (WDs), we have begun an initiative using Hubble Space Telescope's Fine Guidance Sensors (FGS) to resolve suspected binary WDs. With the increasing number of WD trigonometric parallaxes becoming available via CTIO's and the USNO's ongoing parallax programs, we have targeted objects that are overluminous at V magnitude and are presumably unresolved multiple systems. A few targets were selected because of spectral anomalies or possible perturbations evident in the residuals of the trigonometric parallax solutions. A total of 16 HST orbits were designated to this program and 12 are completed. Of the eleven WDs observed thus far (one object was observed twice), all but one were unresolved. Analysis of a recent orbit's data indicate a pair was resolved with a separation of 70 mas and a delta V magnitude of ∼1.4. Coupled with astrometric data from the USNO parallax program, we have obtained preliminary constraints on component masses.