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 M _ { \odot } ) 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 \approx 1 % ) , high-cadence ( \approx 2 s ) , multicolor 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 ( \approx 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 M _ { \odot } ) and surface gravity ( \log ( g ) = 6.32 –6.38 ; radii are 0.0423–0.0433 R _ { \odot } ) constraints do not agree with previous spectroscopic determinations .
We use precise eclipse timing to detect the Rømer delay at 7 \sigma significance , providing an additional weak constraint on the masses and limiting the eccentricity to e \cos \omega = ( -4 \pm 5 ) \times 10 ^ { -5 } .
Finally , we use multicolor data to constrain the secondary ’ s effective temperature ( 7600 \pm 120 K ) and cooling age ( 1.6–1.7 Gyr ) .