  We introduce a very simple model of a dark halo. It is a close relative of
Hernquist's model, being generated by the same transformation but this time
applied to the logarithmic potential rather than the point mass. The density is
proportional to (distance)$^{-1}$ at small radii, whilst the rotation curve is
flat at large radii. Isotropic and radially anisotropic distributions functions
are readily found, and the intrinsic and line of sight kinematical quantities
are available as simple formulae. We also provide an analytical approximation
to the Hamiltonian as a function of the actions. As an application, we study
the kinematic properties of stellar haloes and tracers in elliptical galaxies.
We show that the radial velocity dispersion of a power-law population in a
galaxy with a flat rotation curve always tends to the constant value. This
holds true irrespective of the anisotropy or the lengthscales of the dark or
luminous matter. An analogous result holds for the line of sight or projected
velocity dispersion of a power-law surface brightness profile. The radial
velocity dispersion of Population II stars in the Milky Way is a strongly
declining function of Galactocentric radius. So, if the rotation curve is flat,
we conclude that the stellar halo density cannot follow a power-law at large
radii, but must decrease more sharply (like an Einasto profile) or be abruptly
truncated at large radii. Both the starcount and kinematic data of the Milky
Way stellar halo are well-represented by an Einasto profile with index
$m\approx 2$ and effective radius $\approx 20$ kpc.
