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We present deep XMM observations and ESO WFI optical imaging of twoX-ray-selected fossil group candidates, RXCJ0216.7-4749 and RXCJ2315.7-0222.Using the X-ray data, we derive total mass profiles under the hydrostaticequilibrium assumption. The central regions of RXCJ0216.7-4749 are found to bedominated by an X-ray bright AGN, and although we derive a mass profile,uncertainties are large and the constraints are significantly weakened due tothe presence of the central source. The total mass profile of RXCJ2315.7-0222is of high quality, being measured in fifteen bins from [0.075 - 0.75]R500 andcontaining three data points interior to 30 kpc, allowing comprehensiveinvestigation of its properties. We probe several mass models based on thestandard NFW profile or on the Sersic-like model recently suggested byhigh-resolution N-body simulations. We find that the addition of a stellarcomponent due to the presence of the central galaxy is necessary for a goodanalytical model fit. In all mass profile models fitted, the mass concentrationis not especially high compared to non-fossil systems. In addition, themodification of the dark matter halo by adiabatic contraction slightly improvesthe fit. However, our result depends critically on the choice of IMF used toconvert galaxy luminosity to mass, which leads to a degeneracy between thecentral slope of the dark matter profile and the normalisation of the stellarcomponent. While we argue on the basis of the range of M_*/L_R ratios thatlower M_*/L_R ratios are preferred on physical grounds and that adiabaticcontraction has thus operated in this system, better theoretical andobservational convergence on this problem is needed to make further progess.

Testing adiabatic contraction of dark matter in fossil group candidates