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We report the discovery of an unbound hyper-velocity star, US 708, in theMilky Way halo, with a heliocentric radial velocity of +708+-15km/s. Aquantitative NLTE model atmosphere analysis of optical spectra obtained withLRIS at the Keck I telescope shows that US 708 is an extremely helium-rich(N(He)/N(H)=10) subluminous O type star with Teff=44500K, log g=5.23 at adistance of 19kpc. Its Galactic rest frame velocity is at least 751km/s, muchhigher than the local Galactic escape velocity indicating that the star isunbound to the Galaxy. It has been suggested that such hyper-velocity stars canbe formed by the tidal disruption of a binary through interaction with thesuper-massive black hole (SMBH) at the Galactic centre (GC). Numericalkinematical experiments are carried out to reconstruct the path from the GC.U S708 needs about 32Myrs to travel from the GC to its present position, less thanits evolutionary lifetime. Its predicted proper motion mue(alpha)cos(delta)=-2.3mas/y and mue(delta)=-2.4mas/y should be measurable by futurespace missions. We conjecture that US 708 is formed by the merger of two heliumwhite dwarfs in a close binary induced by the interaction with the SMBH in theGC and then escaped.Comment: 4 pages, 3 figure

US 708 – an unbound hyper-velocity subluminous O star