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We present orbital solutions for 34 double-lined spectroscopic binaries found in the Carney-Latham sample of 1464 stars selected for high proper motion. We use TODCOR, a two-dimensional correlation technique, to extract the velocities for the primary and secondary stars and their light ratio. For our single-order echelle spectra, obtained with the Center for Astrophysics Digital Speedometers, we find that we can reach secondaries that are as much as 2 mag fainter than their primaries. The ratio of the primary to secondary velocity residuals from the orbital fit equals approximately the secondary-to-primary light ratio, as would be expected for the photon-limited case. We use our mass and light ratios to evaluate the mass-luminosity relation for metal-poor main-sequence dwarfs in the mass range 0.55-0.8 M . We assume an L / M relation and find that the exponent at around 5200 A ˚is 7.4 AE 0.6. We find this is in good agreement with the slope of the corresponding theoretical M V -M 14 Gyr isochrones from the VandenBerg & Bell models for metal-poor stars.

In this paper we derive the mass-ratio and secondary-mass distributions of a large, well-defined, complete sample of 129 spectroscopic binaries with periods between 1 and 2500 days. The binaries, whose orbits were published recently, were detected by a systematic radial-velocity survey of a sample of more than 1400 large proper motion stars. Three features stand out in the mass-ratio distribution: a rise as the mass ratio goes down to q $ 0:2, a sharp drop below q $ 0:2, and a smaller peak at q $ 0:8. Another way to characterize the results is to state that the distribution includes two '' populations,'' one with a high asymmetric peak at q $ 0:2 and another with a smaller peak at q $ 0:8, while the minimum between the two populations is centered at q $ 0:55. The size of the binary sample allows us to divide it into two subsamples and look for differences in the mass-ratio distributions of the two subsamples. We performed two different divisions: one into Galactic halo versus disk populations, and the other into high-and low-mass primary stars (above and below 0.67 M ). The former division yields differences with moderate statistical significance of 88%, while the latter is more significant at a level of 97%. Our analysis suggests that the rise toward low mass ratios does not appear in the mass-ratio distribution of the halo binaries. The other separation shows a broad peak at mass ratio of q $ 0:8 1 for the subsample of binaries with low-mass primaries but no corresponding peak in the subsample with high-mass primaries. We discuss our findings and their application to theories of binary formation.

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