A set of FEROS spectra illustrating the combined effect of rotation and high spectral resolution on the central depth of the strategic He I λ4026, 4471 and He II λ4200, 4541 lines, for stars of similar spectral type (as attributed by Walborn) and similar log W′, as measured from our high-resolution data. Note that in the Walborn scheme the O7 subtype is defined by He I λ4471 ≈ He II λ4541 , whereas in our spectra He I λ4471 is always deeper than He II λ4541 (although its EW is actually smaller, log W′ ≤ 0), indicating a spectral type of O8–8.5.
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