Correlations between cosmic strings and extra relativistic species
  The recent observation that the Cosmic Microwave Background (CMB) may prefer
a neutrino excess has triggered a number of works studying this possibility.
The effect obtained by the non-interacting massless neutrino excess could be
mimicked by some extra radiation component in the early universe, such as a
cosmological gravitational wave background. Prompted by the fact that a
possible candidate to source those gravitational waves would be cosmic strings,
we perform a parameter fitting study with models which considers both cosmic
strings and the effective number of neutrinos as free parameters, using CMB and
non-CMB data. We find that there is a correlation between cosmic strings and
the number of extra relativistic species, and that strings can account for all
the extra radiation necessary. In fact, CMB data prefer strings at a 2sigma
level, paying the price of a higher extra radiation component. CMB data also
give a moderate preference for a model with ns=1. The inclusion of non-CMB data
lowers both the preference for strings and for the extra relativistic species.
