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 new Berry phase: a frequency-independent topological polarization rotation of CMB photons equal to the fine structure constant up to a rational number. This manifests itself as a rotation of E-modes in the CMB polarization to B-modes. The current CMB experimental sensitivity to this rotation is about 1%, with many orders of magnitude improvement expected for future experiments. These strings may also be visible in strongly lensed quasar systems. I will show how measuring the undetermined rational number may shed light on the quantization of electric charge in the standard model and how CMB could rule out Grand Unification.