Mass density slope of elliptical galaxies from strong lensing and
  resolved stellar kinematics
  We discuss constraints on the mass density distribution (parameterized as
$\rho\propto r^{-\gamma}$) in early-type galaxies provided by strong lensing
and stellar kinematics data. The constraints come from mass measurements at two
`pinch' radii. One `pinch' radius $r_1=2.2 R_{Einst}$ is defined such that the
Einstein (i.e. aperture) mass can be converted to the spherical mass almost
independently of the mass-model. Another `pinch' radius $r_2=R_{opt}$ is chosen
so that the dynamical mass, derived from the line-of-sight velocity dispersion,
is least sensitive to the anisotropy of stellar orbits. We verified the
performance of this approach on a sample of simulated elliptical galaxies and
on a sample of 15 SLACS lens galaxies at $0.01 \leq z \leq 0.35$, which have
already been analysed in Barnabe et al. (2011) by the self-consistent joint
lensing and kinematic code. For massive simulated galaxies the density slope
$\gamma$ is recovered with an accuracy of $\sim 13\%$, unless $r_1$ and $r_2$
happen to be close to each other. For SLACS galaxies, we found good overall
agreement with the results of Barnabe et al. (2011) with a sample-averaged
slope $\gamma=2.1\pm0.05$. While the two-pinch-radii approach has larger
statistical uncertainties, it is much simpler and uses only few arithmetic
operations with directly observable quantities.
