Lynn Ann Novak, 18, a 1977 graduate of Herndon High School and a former resident of Reston, was found dead March 18 of accidental carbon monoxide poisoning in Newton, Conn.

An assistant Connecticut medical examiner said Miss Novak's body and that of a companion, Joseph Allen Sheffield, also 18, were found in a parked car. The medical examiner said the couple had parked to wait for a friend and had rolled up the windows and left the motor running, apparenly because of the cold winter. He said both deaths were accidental.

Miss Novak lived in Reston for more than three years until last July. Following her graduation from Herndon last spring, Miss Novak had worked briefly as a sales clerk in the Hecht's store at Tysons Corner, Va.

She was the daughter of John P. and E. Rita Novak. The family moved to Danbury, Conn., which is near Newtown, after Capt. Novak retired form the U.S. Marine Corps.

Besides her parents, survivors include a brother, David P., and a sister, Tracy R., also of the home in Danbury, and three grandparents, Peter J. Novak, of Ormond Beach, Fla., and Mr. and Mrs. Harry Mascolo, of Danbury, Conn.