Einstein Radii from Binary-Source Lensing Events

We show that the Einstein ring radius and transverse speed of a lens projected on the source plane, _E and , 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 than the Einstein timescale, P ≫ t_E, there exists a single twofold degeneracy in determining _E. However, when P ≲ t_E the degeneracy can often be broken by making use of the binary-source system's orbital motion. For ~8% of all lensing events with sources V < 20 seen toward the Large Magellanic Cloud (LMC), one can unambiguously determine whether the lenses are Galactic or whether they lie in the LMC itself. In addition, we propose to include eclipsing binaries as sources for gravitational lensing experiments.