  The presence of a white dwarf in a resolved binary system, such as Sirius,
provides an opportunity to combine dynamical information about the masses, from
astrometry and spectroscopy, with a gravitational red-shift measurement and
spectrophotometry of the white dwarf atmosphere to provide a test of
theoretical mass-radius relations of unprecedented accuracy. We demonstrated
this with the first Balmer line spectrum of Sirius B to be obtained free of
contamination from the primary, with STIS on HST. However, we also found an
unexplained discrepancy between the spectroscopic and gravitational red-shift
mass determinations. With the recovery of STIS, we have been able to revisit
our observations of Sirius B with an improved observation strategy designed to
reduce systematic errors on the gravitational red-shift measurement. We provide
a preliminary report on the refined precision of the Sirius B mass-radius
measurements and the extension of this technique to a larger sample of white
dwarfs in resolved binaries. Together these data can provide accurate mass and
radius determinations capable of testing the theoretical mass-radius relation
and distinguishing between possible structural models.
