Determination of the hydrogen and helium mass fractions in star WR24. Left panel: the black line is the observed spectrum. The coloured lines are models in which the amounts of H and He are varied: the ratio He/H (by number) is 0.1 (0.2, 0.4) in the blue (red, green) model. Right panel: χ² function renormalised to the minimum value for hydrogen (red) and helium (blue), as a function of mass fraction. The vertical dashed lines indicate the best-fit values, while the dotted lines mark the error bars.