Top panel: contour diagram showing the carbon-to-iron mass ratio, as a function of progenitor mass and explosion energy, predicted for the supernovae yields of zero metallicity stars by Heger & Woosley (2010). Contours are shown for log  (C/Fe) = 2,4,10 and 20. Bottom panel: the carbon-to-iron ratio, as a function of iron production, predicted for the supernovae models (rhombi; with iron multiplied by an arbitrary factor), and abundances found in the most iron-poor stars known (the upper limits for iron in SMSSJ031300.36−670839.3 have been ignored for simplicity). The asterisks correspond to stars that have [C/Fe] > + 2, and the open circles show those with lower carbon-to-iron ratios.