On the degeneracy between primordial tensor modes and cosmic strings in   future CMB data from Planck
  While observations indicate that the predominant source of cosmic inhomogeneities are adiabatic perturbations, there are a variety of candidates to provide auxiliary trace effects, including inflation-generated primordial tensors and cosmic defects which both produce B-mode cosmic microwave background (CMB) polarization. We investigate whether future experiments may suffer confusion as to the true origin of such effects, focusing on the ability of Planck to distinguish tensors from cosmic strings, and show that there is no significant degeneracy. 
Jon Urrestilla, Pia Mukherjee, Andrew R. Liddle, Neil Bevis, Mark   Hindmarsh, Martin Kunz