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Our work is based on the stacked weak-lensing analysis of a sample of 80strong-lensing galaxy groups. Our main results are the following: (i) thelensing signal does not allow us to firmly reject a simple singular isothermalsphere mass distribution compared to the expected NFW mass profile; (ii) weobtain an average concentration $c_{200}=8.6_{-1.3}^{+2.1}$ that is much higherthan the expected value from numerical simulations for the correspondingaverage mass $M_{200}=0.73_{-0.10}^{+0.11}\times10^{14}\mathrm{M_{\odot}}$;(iii) the combination of our results with those at larger mass scales gives amass-concentration relation $c(M)$ over nearly two decades in mass, with aslope in disagreement with predictions from numerical simulations usingunbiased populations of dark matter haloes; (iv) our combined $c(M)$ relationmatches results from simulations using only haloes with a large strong-lensingcross section, i.e. elongated with a major axis close to the line of sight; (v)for the simplest case of prolate haloes, we estimate with a toy model a lowerlimit on the minor:major axis ratio $a/c=0.5$ for the average SARCS galaxygroup.  Our analysis based on galaxy groups confirmed the results obtained at largermass scales: strong lenses present apparently too large concentrations, whichcan be explained by triaxial haloes preferentially oriented with the line ofsight. Because more massive systems already have large lensing cross sections,they do not require a large elongation along the line of sight, contrary toless massive galaxy groups. Therefore, it is natural to observe larger lensing(projected) concentrations for such systems, resulting in an overallmass-concentration relation steeper than that of non-lensing haloes.

SARCS strong-lensing galaxy groups: II - mass-concentration relation and strong-lensing bias