X-ray satellites have discovered a new and common group "binary" systems in which both stars have low (solar-like) masses. Other properties of them are poorly known largely because they are at great distances from the Sun and in directions where absorption by interstellar gas makes their true brightnesses and hence distances ambiguous. In previous work the Principal Investigator (PI) has discovered the optical counterparts to these new x-ray sources and has monitored their optical output and radial velocities to learn more about their binary characteristics that enable us to understand the reason for their x-ray emission. In the present three year study the PI will study a sample of these objects in the Large Magellanic Cloud (LMC). Because these sources are at a common distance their properties can actually be defined better than those in our own Galaxy. This work is expected to delineate differences between x-ray sources in our Galaxy and the LMC, for example, in terms of differences in heavy metal content. The PI's past work has led to the discovery of the first black-hole in an eclipsing binary (leading to a direct estimate of the mass of the black hole). This work is expected also to clarify the existence of these exotic objects.
