Litton Systems Inc., a division of Litton Industries Inc., agreed to buy Denro Inc. for about $60 million in cash, as the aerospace, defense and commercial electronics company expands its voice and data communications business. Woodland Hills-based Litton said its subsidiary will acquire all stock in Denro, a manufacturer of data recorders used in air traffic control. Gaithersburg, Md.-based Denro, a subsidiary of Canadian holding company Firan Corp., had 1997 sales of $37.8 million. The acquisition will add to Litton's efforts to "bring advanced technical solutions to the worldwide air traffic management market," said Michael R. Brow, Litton's president and chief executive. Litton shares fell $2.25 to close at $61.19 on the New York Stock Exchange.