I demonstrate that an effect similar to the Römer delay , familiar from timing radio pulsars , should be detectable in the first eclipsing double white dwarf ( WD ) binary , NLTT 11748 . By measuring the difference of the time between the secondary and primary eclipses from one-half period ( 4.6 s ) , one can determine the physical size of the orbit and hence constrain the masses of the individual WDs . A measurement with uncertainty < 0.1 s—possible with modern large telescopes—will determine the individual masses to \pm 0.02 M _ { \odot } when combined with good-quality ( < 1 { km s } ^ { -1 } ) radial velocity data , although the eccentricity must also be known to high accuracy ( \pm 10 ^ { -3 } ) . Mass constraints improve as P ^ { -1 / 2 } ( where P is the orbital period ) , so this works best in wide binaries and should be detectable even for non-degenerate stars , but such constraints require the mass ratio to differ from one and undistorted orbits .