EINSTEIN RADII FROM BINARY-SOURCE LENSING EVENTS

We show that the Einstein ring radius and transverse speed of a lens p rojected on the source plane, (r) over cap(E) and (v) over cap, can be determined from the light curve of a binary-source event, followed by the spectroscopic determination of the orbital elements of the source stars. The determination makes use of the same principle that allows one to measure the Einstein ring radii from finite-source effects. For the case when the orbital period of the source stars is much longer t han the Einstein timescale, P much greater than t(E), there exists a s ingle twofold degeneracy in determining <(r)over car>(E). However, whe n P less than or similar to t(E) the degeneracy can often be broken by making use of the binary-source system's orbital motion. For similar to 8% of all lensing events with sources V <20 seen toward the Large M agellanic Cloud (LMC), one can unambiguosly determine whether the lens es are Galactic or whether they lie in the LMC itself. In addition, we propose to include eclipsing binaries as sources for gravitational le nsing experiments.