Recently it has been pointed out that a cosmic background of relativistic axion-like particles ( ALPs ) would be produced by the primordial decays of heavy fields in the post-inflation epoch , contributing to the extra-radiation content in the Universe today .
Primordial magnetic fields would trigger conversions of these ALPs into sub-MeV photons during the dark ages .
This photon flux would produce an early reionization of the Universe , leaving a significant imprint on the total optical depth to recombination \tau .
Using the current measurement of \tau and the limit on the extra-radiation content \Delta N _ { eff } by the Planck experiment we put a strong bound on the ALP-photon conversions .
Namely we obtain upper limits on the product of the photon-ALP coupling constant g _ { a \gamma } times the magnetic field strength B down to g _ { a \gamma } B \gtrsim 6 \times 10 ^ { -18 } \textrm { GeV } ^ { -1 } \textrm { nG } for ultralight ALPs .