The two most widely known patients at St. Elizabeths Hospital -- John Warnock Hinckley Jr., 30, who shot President Reagan, and Leslie deVeau, 42, a socially prominent Washingtonian who shot and killed her young daughter -- are engaged to be married, NBC News said last night. The network said it was relying on unnamed sources at the hospital and at the Justice Department.

St. Elizabeths spokesman Dr. Harold Thomas said "we're not going to be able to confirm or deny" the report. But U.S. District Judge Barrington D. Parker, who presided at the 1982 trial in which Hinckley was found not guilty by reason of insanity, said he heard about it yesterday afternoon while he was in his courtroom.

"I took two calls on the matter from persons in a position to know -- at least one of them was," said Parker,who has continued to hear Hinckley's petitions on the conditions of his confinement."I'm not at liberty to say anything more than that."

Parker added that the news of engagement "raises quite an interesting question . . . Marriage is considered a contract. If two people have been found mentally incompetent, I wonder if that precludes them" from entering into a legal contract.

Members of the Hinckley family and deVeau's attorney neither confirmed nor denied NBC's report. Mark Carlin, deVeau's attorney, refused to say even if his client is currently unmarried. "I'm not going to answer any of your questions or comment on any of this garbage," Carlin said.

Hinckley's parents, Jack and Jo Ann, are out of the country on vacation, according to an aide. Meanwhile, Hinckley's sister, Diane Sims, was nonplused when reached at home in Dallas.

"I have no idea what NBC News is doing," said Sims, who declined to say when she last communicated with her brother.

Hinckley and deVeau, who were both sent to the hospital's maximum security John Howard Pavilion after each was found not guilty by reason of insanity in separate proceedings, have had a widely reported relationship in the hospital. DeVeau was acknowledged in "Breaking Points," the Hinckleys' book about their ordeal after their son shot the president and three others on March 30, 1981, as one of those who have "encouraged us through their letters, prayers and expressions of love."

Last May, when the book came out, the Hinckleys' coauthor Elizabeth Sherrill said, "John had been living in a world with very few contact points with real people, and this, by golly, was a real person who really did exist and responded to him. It couldn't very well be romantic in there, I don't think. It may be romantic in John's feelings. I'm sure that there's love there."

At the time, Diane Sims said, "As little as they are able to be together and see each other, it has been good that somebody outside of the family has been interested in him and cares about him." Family members said then that Hinckley had ended his obsession with actress Jodie Foster.

DeVeau, a former teacher and social worker who was at one time listed in Washington's social register, was ruled mentally incompetent in the 1982 shooting death of her 10-year-old daughter, Erin. She lost her left arm from a self-inflicted gunshot wound. The shootings created a sensation in their fashionable Friendship Heights neighborhood.

"They're two recovering schizophrenics. I think it's pretty sad," said a recently retired hospital official who asked not to be named. He said he had often seen the two together in the John Howard Pavilion but knew nothing about an engagement. "A lot of patients become attached to one another. In their case, I think it's a healthy development."