Relation between the dereddened He II 1640 (abscissa) and He II 4686 line fluxes (ordinate). Solid symbols: Milky Way stars; open symbols: LMC stars. WR 25 and WR 43a are marked with a cross. Dashed line: relation for a constant ratio of R₀ = 7.76; solid line: least-square fit to the data excluding the two marked stars. The cross at the lower right indicates the error of the individual measurements.