Microlens Mass Measurement Using Triple-Peak Events

We show that one can measure the effects of microlens parallax for binary microlensing events with three well-measured peaks, two caustic crossings plus a cusp approach, and hence derive the projected Einstein radius E. Since the angular Einstein radius θE is measurable from finite-source effects for almost any well-observed caustic crossing, triple-peak events can yield the determination of the lens mass M = (c2/4G)EθE. We note that, to a certain extent, rotation of the binary can mimic the effects of parallax, but it should often be possible to disentangle parallax from rotation by making use of the late-time light curve.