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We report orbital parameters for two low-mass, pre-main sequence,double-lined spectroscopic binaries VSB 111 and VSB 126. These systems wereoriginally identified as single-lined on the basis of visible-lightobservations. We obtained high-resolution, infrared spectra with the 10-m KeckII telescope, detected absorption lines of the secondary stars, and measuredradial velocities of both components in the systems. The visible light spectrawere obtained on the 1.5-m Wyeth reflector at the Oak Ridge Observatory, the1.5-m Tillinghast reflector at the F. L. Whipple Observatory, and the 4.5-mequivalent Multiple Mirror Telescope. The combination of our visible andinfrared observations of VSB 111 leads to a period of 902.1+/-0.9 days, aneccentricity of 0.788+/-0.008, and a mass ratio of 0.52+/-0.05. VSB 126 has aperiod of 12.9244+/-0.0002 days, an eccentricity of 0.18+/-0.02, and a massratio of 0.29+/-0.02. Visible-light photometry, using the 0.8-m telescope atLowell Observatory, provided rotation periods for the primary stars in bothsystems, 3.74+/-0.02 days for VSB 111 and 5.71+/-0.07 days for VSB 126. Bothbinaries are located in the young, active star-forming cluster NGC 2264 at adistance of ~800 pc. The difference in the center-of-mass velocities of the twosystems is consistent with the radial velocity gradient seen across NGC 2264.To test the evolutionary models for accuracy and consistency, we compare thestellar properties derived from several sets of theoretical calculations forpre-main sequence evolution with our dynamical results.

Orbital Parameters for the Two Young Binaries VSB 111 and VSB 126