  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.
