  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 ($385\pm79$ km\,s$^{-1}$) than that of US 708 ($998\pm68$
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.

Solar and Stellar Astrophysics