C. David Heymann is alleging - for the third time - that Jacqueline and Bobby Kennedy had an affair, and once again, other Kennedy biographers are slamming the author's claim.

In his new book, "Bobby and Jackie: A Love Story" (available on July 14 from Simon & Schuster's Atria Books), Heymann interviews several on-the-record witnesses who say that the in-laws had a sexual relationship after JFK's assassination in 1963.

David Talbot, author of "Brothers: The Hidden History of the Kennedy Years," refused to even comment on Heymann's tome because he doesn't believe the writer is a credible source on the Kennedy family.