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We have spectroscopically confirmed a brown dwarf mass companion to thehydrogen atmosphere white dwarf NLTT5306. The white dwarf's atmosphericparameters were measured using Sloan Digital Sky Survey and X-Shooterspectroscopy as T_eff=7756+/-35K and log(g)=7.68+/-0.08, giving a mass for theprimary of M_WD=0.44+/-0.04 M_sun, at a distance of 71+/-4 pc with a coolingage of 710+/-50 Myr. The existence of the brown dwarf secondary was confirmedthrough the near-infrared arm of the X-Shooter data and a spectral type ofdL4-dL7 was estimated using standard spectral indices. Combined radial velocitymeasurements from the Sloan Digital Sky Survey, X-Shooter and the Hobby-EberlyTelescope's High Resolution Spectrograph of the white dwarf gives a minimummass of 56+/-3 M_jup for the secondary, confirming the substellar nature. Theperiod of the binary was measured as 101.88+/-0.02 mins using both the radialvelocity data and i'-band variability detected with the INT. This variabilityindicates 'day' side heating of the brown dwarf companion. We also observeH{\alpha} emission in our higher resolution data in phase with the white dwarfradial velocity, indicating this system is in a low level of accretion, mostlikely via a stellar wind. This system represents the shortest period whitedwarf + brown dwarf binary and the secondary has survived a stage of commonenvelope evolution, much like its longer period counterpart, WD0137-349. Bothsystems likely represent bona-fide progenitors of cataclysmic variables with alow mass white dwarf and a brown dwarf donor.

NLTT5306: The shortest Period Detached White Dwarf + Brown Dwarf Binary