  We report a measurement of the shape of the source star in microlensing event
MOA 2002-BLG-33. The lens for this event was a close binary whose
centre-of-mass passed almost directly in front of the source star. At this
time, the source star was closely bounded on all sides by a caustic of the
lens. This allowed the oblateness of the source star to be constrained. We
found that a/b = 1.02^{+0.04}_{-0.02} where a and b are its semi-major and
semi-minor axes respectively. The angular resolution of this measurement is
approximately 0.04 microarcsec. We also report HST images of the event that
confirm a previous identification of the source star as an F8-G2 turn-off
main-sequence star.
