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High-velocity stars in the Galactic halo, e.g. the so-called hyper-velocitystars (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 forthe fastest stars among hot subdwarfs (sdB) in the halo is carried out toidentify HVS, unbound to the Galaxy, and bound population II stars in order toderive a lower limit to the halo mass. The radial velocity measurements wereverified at several telescopes to exclude radial velocity variable stars. Outof 88 stars observed in the follow-up campaign 39 stars were found to haveconstant radial velocities. For twelve of them we measured a proper motionsignificantly different from zero and obtained spectroscopic distances fromquantitative spectral analysis to construct the full 6D phase space informationfor a kinematical study. The programme sdBs can be distinguished into twokinematical groups, one (G1) with low Galactic rotation typical of halo starsand a second one (G2) with rapid retrograde motion. The G1 objects crossed theGalactic plane in the central bulge, whereas the G2 stars did in the outerGalactic disc. J1211+1437 (G2) is a HVS candidate, as it is unbound to theGalaxy if the standard Galactic potential is adopted. We conclude that in theejection scenario G1 stars might have been formed via the slingshot mechanismthat invokes acceleration by tidal interaction of a binary with the centralsupermassive black hole. The G2 stars, however, would originate in theoutskirts of the Galactic disc and not in the central bulge. J1211+1437 is thefirst unbound subdwarf B star, for which we can rule out the slingshotmechanism. Alternatively, we may assume that the stars are old population IIstars and therefore have to be bound. Then the kinematics of J1211+1437 set alower limit of 2 x 10^12 Msun to the mass of the Galactic dark matter halo.

The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars