Observed (dots with error bars) and synthetic spectra (lines) around the Pb line at 4057.8 Å. This Pb line lies in the red wing of a weak ¹²CH feature (from the B-X system), which proved to be underestimated with the carbon abundance derived from the G-band (¹²CH A-X system), as shown in the middle panel. Two hypotheses for the abundances of carbon have therefore been considered here: (1) the nominal abundance log  ϵ(C) = 5.94 (middle panel); and (2) an abundance log  ϵ(C) = 6.20, which best fits the CH 4057.7 Å line (right panel). The left panel shows together the best-fit values for Pb (red curves) and the synthesis with no Pb (black curves) in these two cases. Middle panel: syntheses are shown for log  ϵ(Pb) = −0.15 and + 0.25 (respectively, the best-fit and a conservative upper limit, shown in red), and no Pb, assuming the nominal C abundance. Right panel: log  ϵ(Pb) = −0.25 and + 0.25 (respectively, the best-fit and a conservative upper limit, shown in red), and no Pb, assuming log  ϵ(C) = 6.20.