  Photons can gravitationally scatter off a cosmic string loop and gain or lose
energy. We consider the spectral distortion induced by cosmic string loops
placed in an ambient thermal bath of photons. The fractional deviation from a
thermal spectrum caused by cosmic strings is estimated to scale as (G_N \mu)^2
z^2, where G_N is Newton's constant, $\mu$ is the string tension, and z is the
cosmological redshift after which spectral distortions can survive. This effect
is large enough to potentially be of observational interest.
