Days after comedian Bill Cosby's arrest on sexual assault charges, his wife Camille has learned she must testify against the entertainer in a separate civil case filed by seven women who say he defamed them.

A federal magistrate judge in Massachusetts has rejected arguments by Cosby's wife of almost 52 years, who also has been his business manager, that the deposition would represent an "undue burden".

The deposition is scheduled for next Wednesday (local time), a week after Bill Cosby, 78, was charged in Pennsylvania in the only criminal case brought against the actor, who has been accused by more than 50 women of sexually abusing them in incidents dating back decades.

Cosby is free on $US1 million ($1.37 million) bail, and his lawyer has said the entertainer is not guilty and will not consider a plea bargain.

Cosby, who exemplified the model family man in his long-running hit television series "The Cosby Show," was charged with aggravated indecent assault, which carries a maximum penalty of five to 10 years in prison.

The separate civil charge stems from accusations by Andrea Constand, 44, a former basketball team manager at Temple University in Philadelphia, Cosby's alma mater.

She settled a civil case against Cosby for an undisclosed sum in 2006.

The Massachusetts civil lawsuit against Cosby was filed in December 2014 by Tamara Green, later joined by six other women, who contend that Cosby sexually assaulted or abused them, that each publicly accused Cosby, and Cosby responded by calling the women liars, thus defaming them, court documents said.

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Judge David Hennessy also rejected arguments by Camille Cosby that she lacked any first-hand knowledge of the events at issue, and that the court should protect her from "unnecessary harassment" by limiting the scope of the subpoena.

Lawyers for Cosby plan to appeal against Judge Hennessy's ruling, according to the New York Times.

Cosby sent a Twitter message on Thursday afternoon saying: "Friends and fans, Thank You."

Some responded with messages of support, noting he was innocent until proven guilty.

Others showed scorn, asking: "What about the victims?" and "What 'fans'?"

Reuters