1 Gravitational Microlensing: Black Holes, Planets;

OGLE and other projects are likely to discover first stellar mass black holes and the first planets through gravitational lensing in the next year or two. It is important to have follow-up projects ready, using diverse observing methods. The best for black hole detection would be a measurement of image splitting with VLTI, or any other optical interferometer. Alternative approach is to measure non-linear motion of the light centroid with the HST, or even with a ground based telescope. Every year OGLE detects several very long duration microlensing events brighter than I = 16 mag and K = 14 mag. The two images may be separated by up to 10 mas. Ground based detection of strong caustic crossing planetary events will provide mass ratios and proper motions for the detected systems. For most events photometric parallax needed for mass determination will require a space instrument at least as far as the L2 point, to provide long enough baseline.