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In the inflationary scenarios suggested by string theory, the vacuumfluctuations of the electromagnetic field can be amplified by thetime-evolution of the dilaton background, and can grow large enough to explainboth the origin of the cosmic magnetic fields and of the observed CMBanisotropy. The normalization of the perturbation spectrum is fixed, andimplies a relation between the perturbation amplitude at the COBE scale and thespectral index $n$. Working within a generic two-parameter family ofbackgrounds, a large scale anisotropy $\Delta T/T\simeq 10^{-5}$ is found tocorrespond to a spectral index in the range $n\simeq 1.11 - 1.17$.Comment: 11 pages, LATE

Electromagnetic origin of the CMB anisotropy in string cosmology