Michael Nagle for The New York Times

There is no plaque on the brownstone half a block from Prospect Park that says, “Barack Obama slept here.” Not yet, anyway.

The fact that Mr. Obama and his girlfriend at the time lived on the top floor in the mid-1980s was news on Second Street in Park Slope, Brooklyn, where the 109-year-old house is one in a handsome line of homes stretching toward Eighth Avenue.

The residence was mentioned in an excerpt from a new biography of the president that was posted on the Web site of Vanity Fair magazine on Wednesday. It offers revealing glimpses of Mr. Obama’s dating life in New York.

But Roscoe Robinson, who now shares the top floor of the house with his brother, had not heard about it until a reporter rang the doorbell and showed him a printout of the excerpt.

“You’re kidding,” Mr. Robinson, 21, said.

The revelation would give the block claim to a bigger celebrity than the ones others on the block talk about — the novelist Jonathan Safran Foer, who lives a couple of doors from where President Obama used to live; and former Gov. Hugh L. Carey, who once lived on Prospect Park West.

And it would add to Prospect Park’s claim that a future president used to run there.

“I’m flabbergasted,” said Mary Alice Martinez, a retired schoolteacher who has lived on the block since 1979. “Nobody I’ve ever spoken with mentioned him. No one ever mentioned, ‘He looks familiar.’”

Mr. Robinson’s father, Michael D. Robinson, said there had been “a little apartment on the top floor” when he bought the house in 1994. And as they led the way to where Mr. Obama had lived with the girlfriend, Genevieve Cook, Roscoe Robinson said, “Is it possible that Barack walked on these stairs?”

The biography, “Barack Obama: The Story,” by David Maraniss, to be published next month by Simon & Schuster, says Ms. Cook moved there in 1984. The excerpt in Vanity Fair said Mr. Obama moved in with Ms. Cook, toward the end of the year, after quitting his job at Business International, a firm whose offices were near the United Nations.

They met at a Christmas party in 1983, after Mr. Obama graduated from Columbia University. The book quotes from Ms. Cook’s journals, which describe a long effort to understand Mr. Obama.

“How is he so old already, at the age of 22?” she wrote in one entry. Later, she added, in what sounds like a description of the apparent emotional coolness that would come to confound some of his supporters, “Distance, distance, distance and wariness.”

“The sexual warmth is definitely there — but the rest of it has sharp edges, and I’m finding it all unsettling and finding myself wanting to withdraw from it all,” Ms. Cook wrote in another entry, in February 1984.

“I have to admit that I am feeling anger at him for some reason, multi-stranded reasons. His warmth can be deceptive. Tho he speaks sweet words and can be open and trusting, there is also that coolness — and I begin to have an inkling of some things about him that could get to me.”

Mr. Obama made a reference to Ms. Cook in his memoir, “Dreams From My Father,” but did not identify her by name, saying she was a white woman he had met in New York.

He described taking an unnamed girlfriend to a play by a black playwright, and later quarreling with her. “She couldn’t be black, she said,” Mr. Obama wrote.

Ms. Cook told Mr. Maraniss that this was not a reference to her. Mr. Maraniss wrote that when he asked Mr. Obama about it, the president said it was not Ms. Cook but was “an example of compression” because “I was very sensitive in my book not to write about my girlfriends, partly out of respect for them.”

The advance reading copy of Mr. Maraniss’s book said the brownstone was owned by an employee of the Brooklyn Friends School in Park Slope, where Ms. Cook had been an assistant teacher. The Vanity Fair excerpt said that in the fall of 1984, she was to start “teaching on her own for the first time,” at Public School 133, about a mile away.