Shining ALP dark radiation

String scenarios typically not only predict axionlike particles (ALPs) but also significant amounts of ALP dark radiation originating from the decay of the inflaton or a more general modulus. In this paper, we study the decay of such nonthermally produced relativistic (but massive) ALPs to photons. If the ALPs are sufficiently highly energetic, contribute to Δ N_eff≳O (0.001 ) , and have a mass m_a≳MeV we find that, using observations of x-, and γ -rays, the CMB and BBN, very small values of the ALP-photon coupling, can be probed, corresponding to an origin of this coupling at the string (or even Planck) scale.