  We report orbital parameters for two low-mass, pre-main sequence,
double-lined spectroscopic binaries VSB 111 and VSB 126. These systems were
originally identified as single-lined on the basis of visible-light
observations. We obtained high-resolution, infrared spectra with the 10-m Keck
II telescope, detected absorption lines of the secondary stars, and measured
radial velocities of both components in the systems. The visible light spectra
were obtained on the 1.5-m Wyeth reflector at the Oak Ridge Observatory, the
1.5-m Tillinghast reflector at the F. L. Whipple Observatory, and the 4.5-m
equivalent Multiple Mirror Telescope. The combination of our visible and
infrared observations of VSB 111 leads to a period of 902.1+/-0.9 days, an
eccentricity of 0.788+/-0.008, and a mass ratio of 0.52+/-0.05. VSB 126 has a
period of 12.9244+/-0.0002 days, an eccentricity of 0.18+/-0.02, and a mass
ratio of 0.29+/-0.02. Visible-light photometry, using the 0.8-m telescope at
Lowell Observatory, provided rotation periods for the primary stars in both
systems, 3.74+/-0.02 days for VSB 111 and 5.71+/-0.07 days for VSB 126. Both
binaries are located in the young, active star-forming cluster NGC 2264 at a
distance of ~800 pc. The difference in the center-of-mass velocities of the two
systems is consistent with the radial velocity gradient seen across NGC 2264.
To test the evolutionary models for accuracy and consistency, we compare the
stellar properties derived from several sets of theoretical calculations for
pre-main sequence evolution with our dynamical results.
