PROPERTIES OF AN ECLIPSING DOUBLE WHITE DWARF BINARY NLTT 11748

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 (1.5 yr, we consttain 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 ( approximately 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 sub([middot in circle])) and surface gravity (log(g) = 6.32-6.38; radii are 0.0423-0.0433 R sub([middot in circle])) 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 [omaga] = (-4 + or - 5) x 10 super(-5). Finally, we use multicolor data to consttain the secondary's effective temperature (7600 + or - 120 K) and cooling age (1.6-1.7 Gyr).