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

Either way, Sigmund Freud, scribbling in the pages of 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 long persisted, despite staunch denials by Freud loyalists. The Swiss psychoanalyst Carl Gustav Jung, Freud’s disciple and later his archrival, claimed that Miss Bernays had confessed to an affair to him. (The claim was dismissed by Freudians as malice on Jung’s part.) And some researchers have even theorized that she may have become pregnant by Freud and have had an abortion.

What was lacking was any proof. But a German sociologist now says he has found evidence that on Aug. 13, 1898, during a two-week vacation in the Swiss Alps, Freud, then 42, and Miss Bernays, then 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 that they occupied Room 11. Freud signed the book, in his distinctive Germanic scrawl, “Dr Sigm Freud u frau,” abbreviated German for “Dr. Sigmund Freud and wife.”