Constraints on source and lens parameters from microlensing variability   in QSO 0957+561 A,B
  From regular monitoring of the Double Quasar QSO 0957+561 A,B there is now general agreement on a time delay of about 416 days. This has made it possible to determine the microlensing residual in the light-curve, see Pelt et al. (1998). We have used two significant microlensing features: 1) A ``quiet'' period with a variability less than 0.05 mag lasting about 8 years, and 2) A change in the residual of 0.25 mag during a time interval of about 5 years. The first feature gives a lower limit for the lens mass, 'M', for a given normalized source radius, 'r', whereas the second feature gives an upper limit. We have considered the amount of mass in a continuum to be a free parameter with possible values between 0% (all mass in lenses) and 90%. At a significance level of 1% the mass can only be constrained within a rather wide range (1e-06 M_solar to 5 M_solar). For the radius 'R' of the source an upper limit of 1e16 cm is found, whereas the normalized source radius 'r' is restricted to be smaller than 30. At a level of 10%, however, the range of possible masses is much narrower (2e-03 M_solar to 0.5 M_solar), and the upper limit of R is about 6e15 cm, whereas the value of 'r' is restricted to be less than 2. We have used an effective transverse velocity 'V' equal to 600 km/s. 
S. Refsdal, R. Stabell, J. Pelt and R. Schild