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 with 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.
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