Maybe it was just a Freudian slip. Or a case of hiding in plain sight.

Either way, Sigmund Freud, scribbling in a Swiss hotel register, appears to have left the answer to a question that has titillated scholars for much of the last century: Did he have an affair with his wife's younger sister, Minna Bernays?

Rumors of a romantic liaison between Freud and his sister-in-law, who lived with the Freuds, have persisted despite staunch denials by Freud loyalists.

The Swiss psychoanalyst Carl Gustav Jung, Freud's disciple and later archrival, claimed Bernays had confessed to an affair to him. (The claim was dismissed by Freudians as malice on Jung's part.) Some researchers have even theorized that she may have become pregnant by Freud and had an abortion.

What was lacking was proof. But a German sociologist says he has found evidence that on Aug. 13, 1898, during a vacation in the Swiss Alps, Freud, then 42, and Bernays, 33, put up at the Schweizerhaus, an inn in Maloja, and registered as a married couple, a finding that may cause historians to re-evaluate their understanding of Freud's own psychology.

A yellowing page of the leather-bound ledger shows they occupied Room 11. Freud signed, in his distinctive Germanic scrawl, "Dr Sigm Freud u frau," abbreviated German for "Dr. Sigmund Freud and wife."

"By any reasonable standard of proof, Sigmund Freud and his wife's sister, Minna Bernays, had a liaison," wrote Franz Maciejewski, a sociologist formerly at the University of Heidelberg and a specialist in psychoanalysis, who tracked down the record in August.

The evidence is persuasive enough for Peter Gay, the Freud biographer and longtime skeptic on what he called "the Minna matter," to say he is now inclined to revise his work accordingly. "It makes it very possible that they slept together," he said. "It doesn't make him or psychoanalysis more or less correct."