The Bear Stearns Cos. plans to buy The Helios Group, an independent options trading firm specializing in making markets for electronic exchange-traded derivatives. Terms were not disclosed. Helios has more than 100 employees comprising traders, technology workers and support staff. In a news release, James E. Cayne, Bear Stearns Cos. president and CEO, said acquiring Helios will allow Bear Stearns to expand its electronic trading and market making and give the company high-tech computer equipment for making trades worldwide. Helios now trades futures and options on single stocks, indexes and fixed-income instruments on 11 exchanges.