We have spectroscopically confirmed a brown dwarf mass companion to the hydrogen atmosphere white dwarf NLTT 5306 .
The white dwarf ’ s atmospheric parameters were measured using Sloan Digital Sky Survey and X-Shooter spectroscopy as T _ { eff } = 7756 \pm 35 K and log ( g ) = 7.68 \pm 0.08 , giving a mass for the primary of M _ { WD } = 0.44 \pm 0.04 M _ { \odot } at a distance of 71 \pm 4 pc with a cooling age of 710 \pm 50 Myr .
The existence of the brown dwarf secondary was confirmed through the near-infrared arm of the X-Shooter data and a spectral type of dL4-dL7 was estimated using standard spectral indices .
Combined radial velocity measurements from the Sloan Digital Sky Survey , X-Shooter and the Hobby-Eberly Telescope ’ s High Resolution Spectrograph of the white dwarf gives a minimum mass of 56 \pm 3 M _ { \textrm { \tiny Jup } } for the secondary , confirming the substellar nature .
The period of the binary was measured as 101.88 \pm 0.02 mins using both the radial velocity data and i ^ { \prime } -band variability detected with the INT .
This variability indicates ’ day ’ side heating of the brown dwarf companion .
We also observe H \alpha emission in our higher resolution data in phase with the white dwarf radial velocity , indicating this system is in a low level of accretion , most likely via a stellar wind .
This system represents the shortest period white dwarf + brown dwarf binary and the secondary has survived a stage of common envelope evolution , much like its longer period counterpart , WD 0137 - 349 .
Both systems likely represent bona-fide progenitors of cataclysmic variables with a low mass white dwarf and a brown dwarf donor .