  Observational tests of the white dwarf mass-radius relationship have always
been limited by the uncertainty in the available distance measurements. Most
studies have focused on Balmer line spectroscopy because these spectra can be
obtained from ground based observatories, while the Lyman lines are only
accessible to space based UV telescopes. We present results using parallax data
from Gaia DR2 combined with space based spectroscopy from HST and FUSE covering
the Balmer and Lyman lines. We find that our sample supports the theoretical
relation, although there is at least one star which is shown to be
inconsistent. Comparison of results between Balmer and Lyman line spectra shows
they are in agreement when the latest broadening tables are used. We also
assess the factors which contribute to the error in the mass-radius
calculations and confirm the findings of other studies which show that the
spread in results for targets where multiple spectra are available is larger
than the statistical error. The uncertainty in the spectroscopically derived
log g parameter is now the main source of error rather than the parallax.
Finally, we present new results for the radius and spectroscopic mass of Sirius
B which agree with the dynamical mass and mass-radius relation within 1 sigma.
