Modelling the stellar halo with RR-Lyrae stars
  A seven-parameter distribution function (DF) is fitted to $20\,000$ RR-Lyrae
stars for which only astrometric data are available. The observational data are
predicted by the DF in conjunction with the gravitational potential of a
self-consistent model Galaxy defined by DFs for the dark halo, the bulge and a
four-component disc. Tests of the technique developed to deal with missing
line-of-sight velocities show that adding such velocities tightens constraints
on the DF only slightly. The recovered model of the RR-Lyrae population
confirms that the population is flattened and has a strongly radially biased
velocity distribution. At large radii its density profile tends to $\rho\sim
r^{-4.5}$ but no power law provides a good fit inside the solar sphere. The
model is shown to provide an excellent fit to the data for stars brighter than
$r=16.5$ but at certain longitudes it predicts too few faint stars at
Galactocentric radii $\sim20\mbox{kpc}$, possibly signalling that the halo is
not axisymmetric. The DF is used to predict the velocity distribution of BHB
stars for which space velocities are available. The $z$ components are
predicted successfully but too much anisotropy in the $v_Rv_\phi$ plane is
expected.
