Optical mean (blue) and rms (orange) spectra from the HST/STIS campaign. The rms spectrum is scaled (×5) and shifted in flux (−0.25 × 10⁻¹⁵ erg s⁻¹ cm⁻² Å⁻¹) to highlight weak line features. The linear pseudo-continuum beneath the O Iλ8446 and the Ca II triplet complex is shown for orientation. The inset shows this spectral region in greater detail, with the rms spectrum scaled (×15) and shifted (−1.5 × 10⁻¹⁵ erg s⁻¹ cm⁻² Å⁻¹). The O I λ8446 integration area is shaded in blue (see Sect. 3.2).