DEFOCUSING GRAVITATIONAL MICROLENSING

We introduce the notion of defocusing gravitational lens considering a MACHO located behind a light source with respect to an observer. The consequence of defocusing effect is a temporal variability of star lum inosity which produces a gap instead of a peak as tell-tale signature in the light curve. A general theory of (de)focusing rays (geodesics) in a gravitational field is presented. Furthermore, we give estimation s of the mass of the lens and the optical depth connected to such phen omenon.