LIMITS ON MACHOS FROM MICROLENSING IN THE DOUBLE QUASAR Q0957+561

The light curves of the two images of the double quasar Q0957+561 as o btained by Kundic et al. (1997) are almost identical, except for an ov erall time delay and scaling factor. This allows us to put limits on t he amount of microlensing that took place during the time interval cor responding to the monitoring observations. We perform numerical simula tions in which we model the microlensing behaviour of the (halo of the ) lensing galaxy in the system. We test ''MACHO-masses'' ranging from 10(-8) to 10(-1) M. and quasar sizes from 10(14) to 3 x 10(15) cm. Sta tistically comparing the expected microlensing-induced changes from 10 0 000 simulated light curves over a period of 160 days with the (lack of) observed fluctuations, we can constrain regions in the parameter s pace of MACHO mass and quasar size with various degrees of confidence. In particular, a halo consisting of objects at the low end of our mas s scale can be ruled out with high confidence for a small quasar size. A halo consisting of objects with 10(-2) or 10(-1) M. cannot be ruled out yet, but it should produce MACHO induced fluctuations in future o bservations. We also test halos with only 50% or 25% of the mass in co mpact objects; constraints here are a bit less stringent.