Effective temperatures and lithium abundances of halo turnoff stars

Effective temperatures of 30 turnoff stars with -3.2 {[Fe/H]{ have been derived from the profiles of Balmer lines in high S/N, VLT/UVES spectra. While the systematic error of T_eff may be of the order of 100K, the differential values of T_eff are determined with a one-sigma precision of ∼25K. These precise T_eff values are used in a study of the slope and dispersion of the Li abundance as a function of [Fe/H]. A small, but significant cosmic dispersion in A(Li) appears to be present exemplified by the two very metal-poor stars G64-12 and G64-37.