Direct Detection of the L-Dwarf Donor in WZ Sagittae

Analysis of a large set of phase-resolved $K$-band spectra of the cataclysmic variable WZ Sge shows that the secondary star of this system appears to be an L-dwarf. Previous $K$-band spectra of WZ Sge found that the CO overtone bandheads were in emission. We show that absorption from the $^{\rm 12}$CO${\rm (2,0)}$bandheadofthedonorstarcreatesadipinthe$^{\rm 12}$CO${\rm (2,0)}$ emission feature. Measuring the motion of this feature over the orbital period, we construct a radial velocity curve that gives a velocity amplitude of K${}_{\mathrm{a}\mathrm{b}\mathrm{s}}$ = 520 $\pm$ 35 km s${}^{-1}$, consistent with the previously published values for this parameter.