Chemical composition of halo and disk stars with overlapping metallicities.

High resolution (R=60000), high S/N spectra have been obtained for 13 halo stars and 16 disk stars with 5400<~T_eff_<~6500K, 4.0<~logg<~4.6 and overlapping metallicities in the range -1.3<~[Fe/H]<~-0.5. Equivalent widths of weak FeI and FeII lines are used to determine differential values of T_eff_ and logg. Relative abundances of O, Na, Mg, Si, Ca, Ti, Cr, Fe, Ni, Y and Ba are determined to a precision ranging from 0.02 to 0.07dex. Kinematical data have been collected and used to calculate the stellar orbital parameters, R_max_, the maximum distance from the Galactic center, and z_max_, the maximum distance from the Galactic plane. A group of 8 halo stars have significantly lower [α/Fe] values than disk stars of the same metallicity (α= O, Mg, Si, Ca or Ti). These stars are also underabundant in Na and Ni and maybe in Cr. [Na/Fe] ranges from -0.4 to +0.1, and [Ni/Fe] is surprisingly well correlated with [Na/Fe]. The smallest values of [α/Fe] and [Na/Fe] are found for the stars with the largest values of R_max_ and z_max_. This may indicate that the anomalous halo stars have been accreted from dwarf galaxies with a chemical evolution history different from that of the inner halo and the disk. In any case the data show that abundance ratios in stars are not universal functions of [Fe/H] and that the chemical evolution of the Galaxy at [Fe/H]=~-1.0 is more complicated than assumed in many models. One halo star, HD 106038, is found to be strongly overabundant in Si, Ni, Y and Ba relative to Fe.