We measure the radial velocity curve of the eclipsing detached white dwarf binary NLTT 11748 .
The primary exhibits velocity variations with a semi-amplitude of 273 km s ^ { -1 } and an orbital period of 5.641 hr .
We do not detect any spectral features from the secondary star , or any spectral changes during the secondary eclipse .
We use our composite spectrum to constrain the temperature and surface gravity of the primary to be T _ { eff } = 8690 \pm 140 K and \log g = 6.54 \pm 0.05 , which correspond to a mass of 0.18 M _ { \odot } .
For an inclination angle of 89.9 ^ { \circ } derived from the eclipse modeling , the mass function requires a 0.76 M _ { \odot } companion .
The merger time for the system is 7.2 Gyr .
However , due to the extreme mass ratio of 0.24 , the binary will most likely create an AM CVn system instead of a merger .