Evolution of the free electron fraction x_e with redshift z for different values of the fine structure constant α (top) and of the mass of the electron (bottom). The coloured lines refer to a −5% (blue) and + 5% (red) variation of the constants, while the black line shows the standard case. The decreases at redshift around 6000 and 2000 correspond to the first and second recombination of helium, while the large decrease at redshift 1300 is due to the recombination of hydrogen.