A US woman accusing Bill Cosby of sexual assault is entitled to any documents or emails the comedian's criminal lawyer shared with a former prosecutor who has become a key defence witness, a federal judge has ruled.
Former prosecutor Bruce Castor has stepped forward since Cosby's arrest last year to testify that he had promised Cosby a decade ago that he would never be charged in the case.
A judge refused to dismiss the charges on those grounds, but Cosby appealed and remains free on $US1 million ($A1.30 million) bail.
The ruling on Friday came in the related defamation lawsuit that former Temple University employee Andrea Constand filed against Castor. Castor has said Constand changed her story about her encounter with Cosby at his suburban Philadelphia home in 2004 before she sued Cosby for damages.
