Comparison between the EW of various lines. Top panel: EW(He II 1640) vs. EW(N III 4640+C III 4650). Bottom left panel: EW(N III 4640+C III 4650) vs. EW(C IV 5801-12). Bottom right panel: EW(N III 4640+C III 4650) vs. EW(He II 4686). Circles show the sample sources. Green (with an external black circle), orange, and yellow correspond to sources hosting VMS, WR, and VMS or WR, respectively. Black symbols correspond to sources without VMS and WR stars. Dark (light) gray symbols are sources tagged as “unknown” (“?”). The pink pentagon is R136. The typical uncertainties on EW measurements are shown by the black cross. The green, orange, and yellow crosses correspond to the measurements on the average spectra shown in Figs. C.1–C.3, respectively.