IS UU HERCULIS A POST-AGB STAR?

In order to understand the evolutionary status of the anomaloussupergiantUUHer,theprototypeoftheclassofvari- able supergiants located at high galactic latitudes, we obtained several high-resolution spectra of this star, with the 6m tele- scope, over 5 years. This material was used for a search of possible temporal variations of the radial velocity at different depthsinthephotosphereandforstudyingthechemicalcompo- sition. The average radial velocity Vr 130km=s suggests that UUHerbelongstotheoldpopulationoftheGalaxy.Nosystem- atic dependence of the velocity on depth of the line formation layer or on ionization and excitation potential is observed. The radial velocity of the H absorption differs strongly from the average photospheric velocity. The iron abundance in the photosphere of UUHer is signicantly lower than that of the Sun: (Fe=H) = 1:32. The enhancement of nitrogen relatively to iron content (N=Fe) =0 :40 in combination with the carbon underabun- dance (C=Fe) = 0:30 suggests that only a rst dredge-up episode occurred. The Na content is normal relatively to iron, therefore there is no evidence for dredging-up of Ne-Na cycle products.Theheavys-processmetalsY,Baaredepletedrelative to H and Fe, which again implies that the third dredge-up did not occur. From the high luminosity (logg 1), the large radial ve- locity and the chemical abundance pattern, we conclude that UU Her is a low-mass halo star, but not a post-AGB star.