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Cosmic strings generate vector and tensor modes in the B-channel ofpolarization, as well as the usual temperature power spectrum and E-modepolarization spectrum. We use the power spectrum obtained from high-resolutionNambu-Goto cosmic string simulations together with the Planck and BICEP2likelihoods to explore the degeneracies appearing between cosmic strings andother cosmological parameters in different inflationary scenarios, as well asthe constraints that can be imposed on cosmic strings in each of thesesituations. In standard $\Lambda$CDM, the Planck likelihood yields an upperlimit $G\mu<1.49 \times 10^{-7}$ (95% confidence). We also analyse thepossibility of explaining the BB power spectrum signal recently detected by theBICEP2 probe. We find that cosmic strings alone are able to explain only partof the B-mode polarization signal. Apart from the standard $\Lambda$CDM model,we look at the following non-minimal parameters: the running of the spectralindex, non-zero tensor-to-scalar ratio, additional degrees of freedom($N_{eff}$) and sterile neutrinos. We find that in both Planck and BICEP2scenarios adding $N_{eff}$ induces degeneracies between cosmic strings and$N_{eff}$ and other $\Lambda$CDM parameters. With $N_{eff}$ a largercontribution from cosmic strings is allowed, even favoured, but after combiningwith large-scale structure data, such as BAOs, strings remain stronglyconstrained.

Constraints on the Nambu-Goto cosmic string contribution to the CMB power spectrum in light of new temperature and polarisation data