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 0.48 \mathrm { M _ { \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 0.52 \mathrm { M _ { \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 } \geq M _ { \mathrm { H } } / M _ { \mathrm { WD } } \geq 10 ^ { -4 } ) , implying that the surface hydrogen layers of these white dwarfs are not obviously affected by common envelope evolution .