Example of a W_eq measurement by integration (for VFTS 399). The W_eq is extracted by integration of the red region of the spectrum in the left panel. In the middle panel, this is compared to theoretical W_eq values of the combined complex as predicted by FASTWIND, yielding ε_N and its associated error. The robustness of the measurement is checked twofold: first by comparing a model with the final solution to the observed spectrum (left panel, solid black line). Second, in the right panel, by comparing the abundance measurements that result from adjustment of the integration bounds, varying the width Δλ as function of the adopted width Δλ₀, where the adopted measurement is indicated by the red triangle. The 1σ error on W_eq is calculated through propagation of the error spectrum.