We present high time-resolution spectroscopy of the eclipsing cataclysmic variable SDSS J143317.78+101123.3 obtained with QUCAM2 , a high-speed/low-noise electron-multiplying CCD camera . Littlefair et al .
( 4 ) measured the mass of the secondary star in SDSS J143317.78+101123.3 using a light-curve fitting technique and obtained a value of M _ { 2 } = 0.060 \pm 0.003 M _ { \odot } , making it one of the three first bona-fide detections of a brown-dwarf mass donor in a cataclysmic variable .
In this paper we present a dynamical measurement supporting this important result .
We measured the radial-velocity semi-amplitude of the white dwarf from the motion of the wings of the H \alpha emission line and obtained a figure of K _ { 1 } = 34 \pm 4 km s ^ { -1 } , in excellent agreement with the value of K _ { 1 } = 35 \pm 2 km s ^ { -1 } predicted by Littlefair et al. ’ s model .