Comparison of the observed He II 1640 (abscissa) and He II 4686 equivalent widths (ordinate). Solid symbols: Milky Way stars; open symbols: LMC stars. W-R stars of spectral types WN9−11 are marked with a cross. Dotted line: one-to-one relation; solid line: least-square fit to the data excluding the WN9−11 stars. The cross at the lower right indicates the error of the individual measurements.