Mass measurement of a single unseen star and planetary detection efficiency for OGLE 2007-BLG-050

Aims. We analyze OGLE-2007-BLG-050, a high magnification microlensing event (A?432) whose peak occurred on 2?May, 2007, with pronounced finite-source and parallax effects. We compute planet detection efficiencies for this event in order to determine its sensitivity to the presence of planets around the lens star.Methods. Both finite-source and parallax effects permit a measurement of the angular Einstein radius ?E=0.48?0.01?mas and the parallax ?E=0.12?0.03, leading to an estimate of the lens mass M=0.50?0.14M?and its distance to the observer DL=5.5?0.4?kpc. This is only the second determination of a reasonably precise (<30%) mass estimate for an isolated unseen object, using any method. This allows us to calculate the planetary detection efficiency in physical units (r?,mp), where r?is the projected planet-star separation and?mpis the planet mass.Results. When computing planet detection efficiency, we did not find any planetary signature, i.e. none of the planetary configurations provides a ??2improvement higher than?60, and our detection efficiency results reveal significant sensitivity to Neptune-mass planets, and to a lesser extent Earth-mass planets in some configurations. Indeed, Jupiter and Neptune-mass planets are excluded with a high confidence for a large projected separation range between the planet and the lens star, respectively [0.6?10] and [1.4?4]?AU, and Earth-mass planets are excluded with a?10% confidence in the lensing zone, i.e. [1.8?3.1]?AU.