POLARIZATION AS A TOOL FOR GRAVITATIONAL MICROLENSING SURVEYS

Much interest has been generated recently by the ongoing MACHO, EROS a nd OGLE projects to identify gravitationally lensed stars from the Lar ge Magellanic Cloud and Galactic bulge, and the positive identificatio n of several events (Alcock et al., (1993), Aubourg et al., (1993) and Udalski et al., (1993)). The rate at which such events are found shou ld provide considerable information about the distribution of the low mass compact objects, brown dwarfs etc. responsible for the lensing, a nd their relative importance as a component of the cosmological 'dark matter'. We show measurement of the polarisation of starlight during t hese events can yield considerably more information about the lensing objects than was previously considered possible. Furthermore, the cons ideration of extended sources is shown to have a significant effect on the interpretation of the profiles and statistics of the events.