Fig. 4.
Spectral type calibration (solid red line) from fitting the He Iλ4921/He IIλ5411 equivalent-width ratio of the stars observed with MUSE that were classified from VFTS data (Walborn et al. 2014; Evans et al. 2015). The calibration from Kerton et al. (1999), based on the same He Iλ4921/He IIλ5411 ratio for Galactic O stars, is also shown (dashed grey line).
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