Abstract : This thesis comprises two relatively distinct parts. In the first, we present the analysis of 16 millions light curves collected by EROS II towards different longitudes in the Galactic plan during 7 observational seasons. After creating the reference images and the light curves, we searched for gravitational microlensing effects among the produced light curves by applying a discriminating analysis. Meanwhile, we computed our detection efficiency on simulated light curves. We found 24 candidates that enabled us to measure the optical depths towards our targets. The mean optical depth towards the spiral arms is $<\tau_(GSA)> = 0.38\pm0.08$. Our result is in agreement with the predictions of six Galactic models with a central bar, and favours those which do not contain hidden matter in the disk. Besides statistics, the main limitation in our analysis is the fact that the distances to our target stars are not well known.
In the second part, we present the analysis of spectroscopic data, collected during an international campaign to search for type Ia supernovae, conducted during the 1999 spring, and to which EROS II took part. About twenty type Ia supernovae have been discovered and followed up during that campaign. We reduced their spectroscopic data, which consist of about a hundred spectra. We have developed in that purpose an original program to separate the flux of the supernovae and its host galaxy's. We then carried out a standardisations study on a group of objects including those from Nearby99 and published data. The study performed on these 17 type Ia supernovae allowed us to confirm the interest of spectra for standardisation. It shows in particular that a standardisation from spectra is as efficient as a standradisation based on the supernovae luminosity decline rate.