Program: GS-2019B-Q-116

We propose to obtain Zorro speckle observations of low-metallicity ([Fe/H]< -0.5) binary systems which will have a decisive impact on our current understanding about the role of metallicity on the mass-luminosity relation. All our targets are either known single- or double-line spectroscopic binaries, and have been recently scrutinized by lower-precision speckle observations at very small separations with 4m telescopes. Several objects on our sample already have a few orbital points obtained with Gemini-S&N+DSSI. Our full binary sample spans a large range of [Fe/H], down to -2.0. Gemini observations are needed to compute a high-precision resolved (visual) orbit (hence a system´s mass sum) as well as to obtain better-determined individual magnitudes and colors. These systems all have exquiste Gaia DR2 parallaxes, and most have well-determined mass fractions from the spectroscopy, so, when combined with our Gemini orbits, high-precision individual component masses (and luminosities) will immediately follow. With the full data set, it will not only be possible to make a major contribution to the main-sequence mass-luminosity relation for metal-poor stars, but also to conduct sensitive tests of the details of metal-poor stellar evolution models. For the lowest metallicity members in the sample, component colors and magnitudes will also yield new points on the empirical Population II main sequence, which is important for the calibration of ages and distances to globular clusters and therefore our understanding of early Galaxy evolution.