Important tracers for the dark matter halo of the Galaxy are hypervelocity stars ( HVSs ) , which are faster than the local escape velocity of the Galaxy and their slower counterparts , the high-velocity stars in the Galactic halo .
Such HVSs are believed to be ejected from the Galactic Centre ( GC ) through tidal disruption of a binary by the super-massive black hole ( Hills mechanism ) .
The Hyper-MUCHFUSS survey aims at finding high-velocity potentially unbound hot subdwarf stars .
We present the spectroscopic and kinematical analyses of a He-sdO as well as three candidates among the sdB stars using optical Keck/ESI and VLT ( Xshooter , FORS ) spectroscopy .
Proper motions are determined by combining positions from early-epoch photographic plates with those derived from modern digital sky surveys .
The Galactic rest frame velocities range from 203 km s ^ { -1 } to 660 km s ^ { -1 } , indicating that most likely all four stars are gravitationally bound to the Galaxy .
With T _ { \text } { eff } = 47000 K and a surface gravity of \log g = 5.7 , SDSS J205030.39 - 061957.8 ( J2050 ) is a spectroscopic twin of the hypervelocity He-sdO US 708 .
As for the latter , the GC is excluded as a place of origin based on the kinematic analysis .
Hence , the Hills mechanism can be excluded for J2050 .
The ejection velocity is much more moderate ( 385 \pm 79 km s ^ { -1 } ) than that of US 708 ( 998 \pm 68 km s ^ { -1 } ) .
The binary thermonuclear supernova scenario suggested for US 708 would explain the observed properties of J2050 very well without pushing the model parameters to their extreme limits , as required for US 708 .
Accordingly , the star would be the surviving donor of a type Ia supernova .
Three sdB stars also showed extreme kinematics ; one could be a HVS ejected from the GC , whereas the other two could be ejected from the Galactic disk through the binary supernova mechanism .
Alternatively , they might be extreme halo stars .