Starting with a concise introduction on gravitational lensing, time delays between lensed quasars and
its importance as a cosmological probe to estimate the Hubble constant, we present our contribution to
this domain. In a first part, we explain the details of the Numerical Model Fit, a method to estimate
time delays between two or more lensed quasar images. We apply this technique to the light curves of 11
lensed quasars with known time delays in order to analyse these published delays in a more homogeneous
way. Some results can be confirmed, but others prove to be unreliable.
The second part is devoted to the most recent results of our contribution to the COSMOGRAIL
collaboration. We briefly summarize the data reduction and analysis tools before the presentation of the
photometry and time delay analysis of 6 lensed quasars. On top of the confirmation of time delays in two
objects, SDSS J1206+4332 and SDSS J1650+4251, we are the first to measure the time delay in three
doubly lensed quasar systems: HS 2209+1914, SDSS J0903+5028, and SDSS J1155+6346. The time
delay analysis of the well-known quadruply lensed quasar PG 1115+080 reveals interesting elements,
and asks for further investigation of longer light curves.