  We present high-quality ULTRACAM photometry of the eclipsing detached
double-white dwarf binary NLTT 11748. This system consists of a carbon/oxygen
white dwarf and an extremely-low mass (< 0.2 Msun) helium-core white dwarf in a
5.6 hr orbit. To date such extremely-low mass WDs, which can have thin,
stably-burning outer layers, have been modeled via poorly-constrained
atmosphere and cooling calculations where uncertainties in the detailed
structure can strongly influence the eventual fates of these systems when
mass-transfer begins. With precise (individual precision ~1%) high-cadence (~2
s) multi-color photometry of multiple primary and secondary eclipses spanning
>1.5 yr, we constrain the masses and radii of both objects in the NLTT 11748
system to a statistical uncertainty of a few percent. However, we find that
overall uncertainty in the thickness of the envelope of the secondary
carbon/oxygen white dwarf leads to a larger (~13%) systematic uncertainty in
the primary He WD's mass. Over the full range of possible envelope thicknesses
we find that our primary mass (0.136-0.162 Msun) and surface gravity
(log(g)=6.32-6.38; radii are 0.0423-0.0433 Rsun) constraints do not agree with
previous spectroscopic determinations. We use precise eclipse timing to detect
the Romer delay at 7 sigma significance, providing an additional weak
constraint on the masses and limiting the eccentricity to e*cos(omega)= -4e-5
+/- 5e-5. Finally, we use multi-color data to constrain the secondary's
effective temperature (7600+/-120 K) and cooling age (1.6-1.7 Gyr).
