  The chemical composition of stars with extremely low metal contents (taking
``metals'' to mean all elements other than hydrogen and helium) provides us
with information on the masses of the stars that produced the first metals.
Such a direct connection is not possible, however, if the surface of the star
has been polluted by enriched material, either dredged from the star's interior
or transferred from a companion star. Here we argue that, in the case of
HE0107-5240 (ref. 1), the most iron poor star known, the oxygen abundance could
be a discriminant: a ratio of [O/Fe] exceeding +3.5 would favour a pristine
origin of metals, whereas an [O/Fe] ratio of less than +3 would favour the
pollution hypothesis. Using this criterion, we suggest how the required
information on oxygen abundance might be obtained.
