Hidden Sector Gaugino Condensation and the Model-independent Axion
  In the effective field theory framework, we consider the effect of supersymmetry breaking via gaugino condensation and supergravity in the hidden sector gauge group on the hidden sector vacuum angle $\theta_h$. The $\theta_h$ parameter dependence of the potential yields phenomenologically acceptable invisible axion solutions if the $U(1)_R$ symmetry is broken down to a discrete subgroup $Z_N$ with $N\ge 3$ (N=4 is marginal). Anomalous U(1) superstring models are good candidates for this invisible axion resolution of the strong CP puzzle. 
Howard Georgi, Jihn E. Kim, and Hans-Peter Nilles