Microlensing is the only known direct method to measure the masses of stars that lack visible companions. By quantitatively assessing the effects of the extent of the source star and the annual parallax seen in the photometric light curve, the combined efforts by the OGLE, PLANET, and MicroFUN collaborations with the 1.3m Warsaw telescope at Las Campanas Observatory (Chile), the CTIO 1.3m at Cerro Tololo (Chile), the Wise 1.0m at Mitzpe Ramon (Israel), the Danish 1.54m at ESO La Silla (Chile) and the Canopus 1.0m near Hobart (Tasmania) not only allowed to measure the event timescale tE = (37.6 ± 0.2) d and the angular Einstein radius &thetaE = (652 ± 56) μas, but also revealed a 1-σ constraint on the parallax, given by 0.056 < πLS/&thetaE < 0.23, so that the lens mass is constrained to 0.36 Msun < M < 1.48 Msun. Although a measurement of the lens mass by this technique is complicated by the fact that those events that are most likely to show prominent finite source effects are the least likely to show prominent parallax effects, vice versa, the data allowed to obtain a meaningful constraint for an event with a relative proper motion between lens and source of μ ∼ 6 mas/yr, which is rather typical for disk lenses. While OGLE 2003-BLG-238 shows the general feasibility of this approach, events with smaller proper motions will provide better opportunities.

The figure shows the peak region of the event with OGLE I, μFUN Chile I, μFUN Chile V, μFUN Israel I, PLANET Tasmania I, and PLANET Chile R data, as well as model lightcurves for the best-fit nonparallax finite-source model (solid) and a point-source model with otherwise identical parameters (dashed).