  High-velocity stars in the Galactic halo, e.g. the so-called hyper-velocity
stars (HVS), are important tracers of the properties of the dark matter halo,
in particular its mass. Based on the SDSS DR6 spectral database a search for
the fastest stars among hot subdwarfs (sdB) in the halo is carried out to
identify HVS, unbound to the Galaxy, and bound population II stars in order to
derive a lower limit to the halo mass. The radial velocity measurements were
verified at several telescopes to exclude radial velocity variable stars. Out
of 88 stars observed in the follow-up campaign 39 stars were found to have
constant radial velocities. For twelve of them we measured a proper motion
significantly different from zero and obtained spectroscopic distances from
quantitative spectral analysis to construct the full 6D phase space information
for a kinematical study. The programme sdBs can be distinguished into two
kinematical groups, one (G1) with low Galactic rotation typical of halo stars
and a second one (G2) with rapid retrograde motion. The G1 objects crossed the
Galactic plane in the central bulge, whereas the G2 stars did in the outer
Galactic disc. J1211+1437 (G2) is a HVS candidate, as it is unbound to the
Galaxy if the standard Galactic potential is adopted. We conclude that in the
ejection scenario G1 stars might have been formed via the slingshot mechanism
that invokes acceleration by tidal interaction of a binary with the central
supermassive black hole. The G2 stars, however, would originate in the
outskirts of the Galactic disc and not in the central bulge. J1211+1437 is the
first unbound subdwarf B star, for which we can rule out the slingshot
mechanism. Alternatively, we may assume that the stars are old population II
stars and therefore have to be bound. Then the kinematics of J1211+1437 set a
lower limit of 2 x 10^12 Msun to the mass of the Galactic dark matter halo.
