  We present high precision, model independent, mass and radius measurements
for 16 white dwarfs in detached eclipsing binaries and combine these with
previously published data to test the theoretical white dwarf mass-radius
relationship. We reach a mean precision of 2.4 per cent in mass and 2.7 per
cent in radius, with our best measurements reaching a precision of 0.3 per cent
in mass and 0.5 per cent in radius. We find excellent agreement between the
measured and predicted radii across a wide range of masses and temperatures. We
also find the radii of all white dwarfs with masses less than 0.48M$_\odot$ to
be fully consistent with helium core models, but they are on average 9 per cent
larger than those of carbon-oxygen core models. In contrast, white dwarfs with
masses larger than 0.52M$_\odot$ all have radii consistent with carbon-oxygen
core models. Moreover, we find that all but one of the white dwarfs in our
sample have radii consistent with possessing thick surface hydrogen envelopes
($10^{-5} \ge M_\mathrm{H}/M_\mathrm{WD} \ge 10^{-4}$), implying that the
surface hydrogen layers of these white dwarfs are not obviously affected by
common envelope evolution.
