A man and a woman trapped by flames on an 11th floor ledge of their luxury apartment building screamed for help and then leaped to their deaths with their pet dog, authorities said yesterday. A third resident died of smoke inhalation.

The couple was driven to the outside ledge as flames ate through the floor of their apartment of Bunker Hill Towers, a downtown complex. Fire officials said new fire alarms apparently malfunctioned and failed to rouse some sleeping tenants.

The bodies of the couple were found in a crumpled, bloody heap with the body of their dog in a courtyard on the east side of the complex.

Fire Department spokesmen said arson had been ruled out as a cause of the 3 a.m. blaze and that a cigarette left smoldering in overstuffed furniture was suspected of starting the fire.

A third victim, Sidney Karton, 44, apparently was overcome by smoke in a hallway after he fled his apartment. His body was found in the 11th floor hall.

"Had he stayed in his room, as 99 percent of the people did, he may have survived," said fire Capt. Tony Di Domenico.

Firefighters apparently did not hear the dead couple's screams.