Harvard police yesterday warned students to be on the watch for a homeless man who might try to enter their dorms, and rooms, trying to befriend them.

Over the weekend, a homeless man conned students into letting him spend the night in Hollis Hall, prompting the warning.

The alleged con artist used the alias Seamus O'Reilly. Harvard University Police (HUPD) described O'Reilly as a 6-foot white male with brown hair, who spoke with an Irish accent.

The con started in early October, when the man approached a first-year in the Yard and identified himself as a first-year Harvard Law School student. The two chatted and ended up exchanging e-mail addresses.

O'Reilly said he was from a wealthy Irish family and owned an apartment on Beacon Hill. He said that his workload often prevented him from making the trip home.

Last week, Harvard police received reports of a man matching O'Reilly's description using computers in the Science Center without permission.

The first-year, who asked not to be identified, exchanged e-mails with O'Reilly over the course of a week. One day, when the student returned to her room, O'Reilly was outside. She invited him up, and he ended up taking a nap for several hours.

"It was somewhat of a flim-flam," said HUPD spokesperson Peggy A. McNamara.

The con came to an end Saturday night, after the student and a friend went out to dinner with O'Reilly.

His frequent presence in the dorm had attracted the attention of other Hollis residents, one of whom contacted the dorm's proctor, who then called the Harvard police.

HUPD officers were waiting at Hollis when the trio returned from dinner. At the time, O'Reilly was wearing a Harvard Business School T-shirt. According to McNamara, con artists who prey on students often wear school-related apparel.

"It seems to help students think that they belong," she said.

Police said they issued O'Reilly a verbal trespass warning. If he is found on campus again, he risks arrest.

The victim said she felt O'Reilly had taken advantage of her, but said that if the same progression of events occurred again, she would act the same way.

"The worst thing he did was mislead," the student said. She said she did notice that her student ID card was missing.

"We presume he won't come back," McNamara said. "But if he does, watch out and call us."