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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.

Aims. This is the first paper of a series devoted to studying the population of blue massive stars in NGC?55, a galaxy of the Sculptor group at a distance of about 2?Mpc.Methods. We have obtained optical (3300-6210??), low-resolution spectra of approximately 200 blue massive stars with VLT-FORS2, which we have classified with the aid of Milky Way and Magellanic Cloud standard stars.Results. We present the first census of massive blue stars in NGC?55. A study of stellar radial velocities shows agreement with existing Hi?rotational velocity curve work and reveals the presence of one object with peculiar velocity. A qualitative study of the stellar metallicity suggests that its global distribution over NGC?55 is close to that of the LMC, as derived from previous studies.Conclusions. We present a catalogue with 164 classifications of blue massive stars in NGC?55. This catalogue is a first and necessary step for the subsequent quantitative study of blue massive stars in NGC?55 with state-of-the-art model atmospheres.

We present results of the photometric campaign for planetary and low-luminosity object transits conducted by the OGLE survey in the 2005 season (Campaign #5). About twenty of the most promising candidates discovered in these data were subsequently verified spectroscopically with the VLT/FLAMES spectrograph. One of the candidates, OGLE-TR-211, reveals clear changes of radial velocity with a small amplitude of 82?m/s, varying in phase with photometric transit ephemeris. Further analysis confirms the planetary nature of this system. Follow-up precise photometry of OGLE-TR-211 with VLT/FORS, together with radial velocity spectroscopy, supplemented with high-resolution, high S/N VLT/UVES spectra allowed us to derive parameters of the planet and host star. OGLE-TR-211b is a hot Jupiter orbiting an F7-8 spectral type dwarf star with a period of 3.68?days. The mass of the planet is equal to 1.03?0.20 MJup, while its radius 1.36+0.18-0.09RJup. The radius is about 20% larger than the typical radius of hot Jupiters of similar mass. OGLE-TR-211b is, then, another example of inflated hot Jupiters ? a small group of seven exoplanets with large radii and unusually low densities ? objects that are a challenge to the current models of exoplanets.