Context : The Strong Lensing Legacy Survey ( SL2S ) provides a sample of strong lensing events associated with massive distant galaxies , some of which lie in the outskirts of galaxy groups and clusters .

Aims : We investigate the internal structure of early-type galaxies in overdense environments , where tidal forces are expected to alter dark matter halos of infalling galaxies .

Methods : SL2SJ02176-0513 is a remarkable lens for the presence of two multiply-imaged systems at different redshifts lensed by a foreground massive galaxy at z _ { lens } = 0.656 : a bright cusp arc at z _ { arc } = 1.847 and an additional double-image system at an estimated redshift of z _ { dbl } \sim 2.9 based on photometry and lensing geometry .
The system is located about 400 kpc away from the center of a massive group of galaxies .
Mass estimates for the group are available from X-ray observations and satellite kinematics .
Multicolor photometry provides an estimate of the stellar mass of the main lens galaxy .
The lensing galaxy is modeled with two components ( stars and dark matter ) , and we include the perturbing effect of the group environment , and all available constraints .

Results : We find that classic lensing degeneracies , e.g .
between external convergence and mass density slope , are significantly reduced with respect to standard systems and infer tight constraints on the mass density profile : ( i ) the dark matter content of the main lens galaxy is in line with that of typical galaxies f _ { dm } ( < R _ { e } ) = 0.41 ^ { +0.09 } _ { -0.06 } ; ( ii ) the required mass associated with the dark matter halo of the nearby group is consistent with X-ray and weak-lensing estimates ( \sigma _ { grp } = 550 ^ { +130 } _ { -240 } ) ; ( iii ) accounting for the group contribution in the form of an external convergence , the slope of the mass density profile of the main lens galaxy alone is found to be \alpha = -1.03 ^ { +0.22 } _ { -0.16 } , consistent with the isothermal ( \alpha = -1 ) slope .

Conclusions : We demonstrate that multiple source plane systems together with good ancillary dataset can be used to disentangle local and environmental effects .