Predicting the second caustic crossing in binary microlensing events

We fit binary lens models to the data covering the initial part of real mic rolensing events in an attempt to predict the time of the second caustic cr ossing. We use approximations during the initial search through the paramet er space for light curves that roughly match the observed ones. Exact metho ds for calculating the lens magnification of an extended source are used wh en we refine our best initial models. Our calculations show that the reliab le prediction of the second crossing can only be made very late, when the l ight curve has risen appreciably after the minimum between the two caustic crossings. The best observational strategy is therefore to sample as freque ntly as possible once the light curve starts to rise after the minimum.