...1,2,

Present address: Max-Planck Institut f $\ddot{\rm u}$r Extraterrestrische Physik, Postfach-1312, 85741, Garching, Germany.

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... (2)

Note that both scales rely on model atmospheres: one is related to observations of a sample of "real'' massive stars analysed through spectroscopy with model atmospheres (hence the name "observational''), whereas the other one is only based on the model grid without direct link to existing stars (hence the name "theoretical'').

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