The Transverse Peculiar Velocity of the Q2237+0305 Lens Galaxy and the
  Mean Mass of Its Stars
  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 (68%
confidence) and find a preferred direction to the East. The mean stellar mass
estimate including a well-defined velocity prior is 0.12 <= <M/Msun> <= 1.94 at
68% confidence, with a median of 0.52 Msun. 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.
