Résumé
The recent discovery of systematic trends of elemental abundances with evolutionary stage in the globular cluster NGC 6397 [1] has given support to the theoretical expectation that the abundances of elements in the atmospheres of unevolved halo stars are significantly affected by atomic diffusion. HE 1327-2326, one of the two known hyper-metal-poor (i.e., [Fe/H] < -5) stars, is in a near-turnoff stage of evolution [2, 3] and seemingly has no lithium (log epsilon(Li) < 0.7, [4]). Based on various techniques we derive improved stellar parameters for this star. In particular, we derive a spectroscopic surface gravity based on the calcium ionization equilibrium in non-LTE [5]. HE 1327-2326 is found to be a subgiant. Calculations show that, while atomic diffusion may have significantly altered the surface abundances of this star, the large deficit of lithium (relative to other metal-poor stars and the amount produced in Big-Bang nucleosynthesis) cannot be explained in this way.