Axion String Constraints
  We study the production of cosmological axions in the standard scenario in which a global string network forms at the Peccei-Quinn phase transition. We make detailed calculations of the axions produced by string loops (a previously overlooked source), comparing these with estimates of other contributions from long strings and domain walls. We dilineate key uncertainities in the string network evolution, the chief of which is the magnitude of the radiative backreaction. We conclude that axions produces by the these topological contributions provide the significant cosmological constraint on the symmetry breaking scale $\fa$ and the mass $\ma$ 
R.A.Battye and E.P.S.Shellard (DAMTP Cambridge)