Using 11-years of OGLE V-band photometry of Q2237+0305 , we measure the transverse velocity of the lens galaxy and the mean mass of its stars .
We can do so because , for the first time , we fully include the random motions of the stars in the lens galaxy in the analysis of the light curves .
In doing so , we are also able to correctly account for the Earth ’ s parallax motion and the rotation of the lens galaxy , further reducing systematic errors .
We measure a lower limit on the transverse speed of the lens galaxy , v _ { t } > 338 { km s ^ { -1 } } ( 68 % confidence ) and find a preferred direction to the East .
The mean stellar mass estimate including a well-defined velocity prior is 0.12 \leq \left < M / M _ { \sun } \right > \leq 1.94 at 68 \% confidence , with a median of 0.52 ~ { } M _ { \sun } .
We also show for the first time that analyzing subsets of a microlensing light curve , in this case the first and second halves of the OGLE V-band light curve , give mutually consistent physical results .