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 (r) over tilde (E). Since the angular Einstein radius theta (E) 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 = (c(2)/4G)(r) over tilde (E)theta (E). 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.