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We report the discovery of a 16th magnitude star, HE0437-5439, with aheliocentric radial velocity of +723+-3km/s. A quantitative spectral analysisof high-resolution optical spectra obtained with the VLT and the UVESspectrograph shows that HE0437-5439 is a main sequence B-type star withTeff=20350K, log g=3.77, solar within a factor of a few helium abundance andmetal content, rotating at v sin i=54km/s. Using appropriate evolutionarytracks we derive a mass of 8 Msun and a corresponding distance of 61 kpc. Itsgalactic rest frame velocity is at least 563km/s, almost twice the localGalactic escape velocity, indicating that the star is unbound to the Galaxy.Numerical kinematical experiments are carried out to constrain its place ofbirth. It has been suggested that such hyper-velocity stars can be formed bythe tidal disruption of a binary through interaction with the super-massiveblack hole at the Galactic center (GC). HE0437-5439 needs about 100Myrs totravel from the GC to its presentposition, much longer than its main sequencelifetime of 25Myrs. This can only be reconciled if HE0437-5439 is a bluestraggler star. In this case, the predicted proper motion is so small that itcan only be measured by future space missions. Since the star is much closer tothe Large Magellanic Cloud (LMC, 18kpc) than to the GC, it can reach itsposition from the center of the LMC. The proper motion predicted in this caseis about 2mas/y (relative to the LMC), large enough to be measurable withconventional techniques from the ground. The LMC origin could also be tested bya high-precision abundance analysis.Comment: 13 pages, 4 figures. Astrophysical Journal Letters, accepte

HE 0437-5439: An Unbound Hypervelocity Main-Sequence B-Type Star