It is obviously interesting to study in detail the chemical composition of extreme Population II stars since they may give indications about the processes which took place in the very first ages of the Galaxy.

It is well known that de abundance of metals in the stars of extreme Population II are smaller than in Population I stars, but it is not yet clear from the literature if all the metals, in all the metal deficient stars, are deficient by the same factor or not. However some anomalies were shown in some halo stars : over-deficiency of barium, “s process” element (Pagel 1965), over-deficiency of odd elements (Arnett 1971, Peterson 1976). These differences are however difficult to measure and are not very much larger than the possible errors of the analysis, so that according to Trimble 1975 they are “not proven”.