Relative uncertainty in EW in function of line strength for different S/N. The line strengths of our five Fe II lines in the TOP stars are also indicated in the plot. The dot-dashed lines represent the theoretical errors in EW following Norris et al. (2001) at λ = 5600 Å. On the right axis the corresponding uncertainties in abundance for weak lines are given. (This figure is available in colour in electronic form.)