Observation of microlensing toward the galactic spiral arms. EROS II 3 year survey *

We present an analysis of the light curves of 9.1 million stars observed during three seasons by EROS(Expérience de Recherche d'Objets Sombres), in the Galactic plane away from the bulge. Seven stars exhibit luminosity variations compatible with gravitational microlensing effects due to unseen objects. The corresponding optical depth, averaged over four directions, is $\bar\tau = 0.43 \pm 0.2 \times 10^{-6}$. While this value is compatible with expectations from simple Galactic models under reasonable assumptions on the target star distances, we find an excess of events with short timescales toward the direction closest to the Galactic centre. We discuss a possible interpretation involving the contribution of an elongated bar.