  Determination of microlensing parameters in the gravitationally lensed quasar
Q2237+0305 from the statistics of high magnification events will require
monitoring for more than 100 years (Wambsganss, Paczynski & Schneider 1990).
However we show that the effective transverse velocity of the lensing galaxy
can be determined on a more realistic time-scale through consideration of the
distribution of light-curve derivatives. The 10 years of existing monitoring
data for Q2237+0305 are analysed. These data display strong evidence for
microlensing that is not associated with a high magnification event. An upper
limit of v < 500 km/sec is obtained for the galactic transverse velocity which
is smaller than previously assumed values. The analysis suggests that the
observed microlensing variation may be predominantly due to stellar proper
motions. The statistical significance of the results obtained from our method
will be increased by the addition of data points from current and future
monitoring campaigns. However reduced photometric errors will be more valuable
than an increased sampling rate.
