Can Impurity Effects Help to Identify the Symmetry of the Order   Parameter of the Cuprates?
  The effects of non-magnetic impurities on the properties of superconductors with order parameters (OP) of different symmetry are discussed. In particular, we contrast the case of a d_{x^2 - y^2} with various forms of an anisotropic s-wave (ASW) gap. The biggest qualitative difference occurs if the phase of the s-wave OP does not change sign. 
R. Fehrenbacher and M.R. Norman