Comparison of the best fits of (upper panel): a single Gaussian shape (black lines) and an Edgeworth shape (blue lines); (middle panel): Edgeworth (black lines) and double Lorentzian shape (blue lines); (lower panel): Edgeworth (black lines) and Gauss-Hermite shape (blue lines) for Observation 4 plotted in the observed frame. Continuous lines show the total model spectrum, dashed lines show the separate components. Differences are relatively small in the upper two panels but the difference in χ² of the fit is significant (see Table 3); the Gauss-Hermite expansion gives a different decomposition since it requires a different Fe II template (see text).