Determination of stellar shape in microlensing event MOA 2002-BLG-33

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 μ{arcsec}. 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.