Résumé : Strong lensing (SL) has been employed extensively to obtain accurate mass measurements within the Einstein radius, i.e. at the location of the multiply imaged systems used as constraints. I will show that SL can be used to probe mass distributions on scales much larger than the one defined by the Einstein radius. I will discuss this method on SL2SJ08544-0121, a galaxy group at redshift z=0.35. This group displays a bimodal light distribution with a strong lensing system located at one of the two luminosity peaks separated by  ~54 arcsec. The main arc and the counter-image of the strong lensing system are located at ~5 arcsec and ~8 arcsec from the lens galaxy centre. I will show that a simple elliptical isothermal potential cannot satisfactorily reproduce the strong lensing observations. However, with a mass model for the group built from its light-distribution, i obtain an accurate reproduction of the strong lensing observations, therefore setting constraints on the mass of the galaxy group as a whole. The SL only analysis hints that we are actually witnessing the merging of two galaxy  groups.
Using complementary data sets, spectroscopy of galaxy group members and Xray observations, i will show that the predictions from the SL only analysis are confirmed. This kind of analysis provides a quick and cheap way of probing the mass distribution of clusters and groups. This is particularly relevant in the context of forthcoming wide field surveys, which will yield thousands of strong lenses.