  We present the results of a high-resolution spectroscopic monitoring program
of the eclipsing pre-main-sequence star KH 15D that reveal it to be a
single-line spectroscopic binary. We find that the best-fit Keplerian model has
a period P = 48.38 days, which is nearly identical to the photometric period.
Thus, we find the best explanation for the periodic dimming of KH 15D is that
the binary motion carries the currently visible star alternately above and
below the edge of an obscuring cloud. The data are consistent with the models
involving an inclined circumstellar disk, as recently proposed by Winn et al.
(2004) and Chiang & Murray-Clay (2004). We show that the mass ratio expected
from models of PMS evolution, together with the mass constraints for the
visible star, restrict the orbital eccentricity to 0.68 < e < 0.80 and the mass
function to 0.125 < Fm < 0.5 Msun.
