Observation of microlensing towards the galactic spiral arms - EROS II. 2 year survey

We present the analysis of the light curves of 8.5 million stars observed d uring two seasons by EROS (Experience de Recherche d'Objets Sombres), in th e Galactic plane away from the bulge. Three stars have been found that exhi bit luminosity variations compatible with gravitational microlensing effect s due to unseen objects. The corresponding optical depth, averaged over fou r directions, is <(tau)over bar> = 0.38(-0.15)(+0.53): x 10(-6). All three candidates have long Einstein radius crossing times (similar to :70 to 100 days). For one of them, the lack of evidence for a parallax or a source siz e effect enabled us to constrain the lens-source configuration. Another can didate displays a modulation of the magnification, which is compatible with the lensing of a binary source. The interpretation of the optical depths inferred from these observations i s hindered by the imperfect knowledge of the distance to the target stars. Our measurements are compatible with expectations from simple galactic mode ls under reasonable assumptions on the target distances.