Spectrum showing the rms variability of Mrk 110, computed across the six RGS observations in the O band. Upper panel: rms spectrum in counts space, compared to a power-law component (black line) for the variable continuum. Lower panel: ratio of the rms spectrum to the powerlaw model, where an excess is visible in the O VII emission band between 0.54–0.57 keV. This confirms the line variability across all six spectra, as is presented in the main article.