Probing the inner structure of quasars and the expansion of the Universe with monitoring of gravitationally lensed quasars

Photometric monitoring of lensed quasars is now performed by several teams around the world in order to measure the differential time-delays between the quasar lensed images. We will present the first results of one of the most ambitious of these program, COSMOGRAIL, which aims to measure time delays in among 20 gravitationally lensed systems with a 5\% accuracy. We will briefly comment about the technical difficulties associated with accurate flux measurements of point-like images separated by a few arcsec. Afterwards, we will discuss the constraints provided by the time delay measurements on the expansion rate of the Universe and on the mass distribution of lensing galaxies. Monitoring lensed quasars also allows to track for extrinsic flux variations due to the stars in the lensing galaxy, the so called quasar microlensing effect. We will explain why this effect has a chromatic signature and how the analysis of spectra of lensed images allows to probe the inner regions of quasars. We will then present the constraints on the size and geometry of the continuum and of the broad line regions from the analysis of single and multi-epoch spectra of two lensed quasars