Detached eclipsing binaries as primary distance and age indicators
  Detached eclipsing double line spectroscopic binaries offer an opportunity to measure directly stellar parameters: mass, luminosity, radius, as well as the distance. The only non-trivial step is the need to determine surface brightness of each component on the basis of something measurable, like the color or the line ratios. Modern model atmospheres provide a fairly good calibration of that relation, but empirical verification is possible, and it is needed to achieve the highest accuracy. When this approach is fully developed the detached eclipsing binaries should provide direct (single step) distances with about 1% accuracy to all galaxies in the Local Group. Recent discovery of numerous detached eclipsing binaries near the main sequence turn-off points in several globular clusters (Kaluzny et al. 1996a,b) will be followed by accurate determinations of distances, masses and luminosities. The empirical the mass - luminosity relation near the main sequence turn-off point will allow accurate age and helium determinations, and a check on the stellar evolution theory. 
Bohdan Paczynski (Princeton University Observatory)