Abstract: Very light axions are a generic prediction of string compactifications. If cosmic strings associated with these axions were produced in the early universe, they quickly approach a so-called scaling solution, such that strings persist in the sky today. I will present some remarkable signals of such strings coupled to photons. In this string background, there is a model-independent polarization rotation of CMB photons equal to alpha_em up to a rational number. This manifests itself as a rotation…