Normalised data (blue crosses) and the O VIII line models at the upper 1-σ limit (red areas) at the redshift of A3 for RGS1 (left panel) and LETG/HRC-S (right panel). The normalisation is done by dividing the spectra by the best-fit model consisting of the PKS 2155–304 emission and all (neutral + ionised) Galactic absorbers. The vertical dashed lines indicate the centroid wavelength of the O VIII doublet redshifted to z = 0.0544. The green lines indicate the lower limit reported by Fang et al. (2007). The error bars contain both the statistical uncertainties and the systematic one (2% of the flux).