  I demonstrate that an effect similar to the Roemer 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 +/-0.02 Msun when
combined with good-quality (<1 km/s) radial velocity data, although the
eccentricity must also be known to high accuracy (+/- 1e-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.
