  Cosmological N-body simulations predict that dark matter halos should have a
universal shape characterized by a steep, cuspy inner profile. Here we report
on a spectroscopic study of six clusters each containing a dominant brightest
cluster galaxy (BCG) with nearby gravitational arcs. Three clusters have both
radial and tangential gravitational arcs, whereas the other three display only
tangential arcs. We analyze stellar velocity dispersion data for the BCGs in
conjunction with the arc redshifts and lens models to constrain the dark and
baryonic mass profiles jointly. For those clusters with radial gravitational
arcs we were able to measure precisely the inner slope of the dark matter halo
and compare it with that predicted from CDM simulations.
