El Salvador's President Jose Napoleon Duarte and rebel leaders will participate in peace talks scheduled to begin on Sunday, a Roman Catholic bishop said. "The dialogue is assured," Msgr. Gregorio Rosa Chavez, auxiliary bishop of San Salvador, told reporters at Metropolitan Cathedral. He said an agenda has not yet been set, but "a central theme will be everything that has to be done to arrive at an end to hostilities." The negotiations, which will be held at the papal nuncio's residence in the capital, will be the first in nearly three years and were prompted by a Central American peace plan signed last month in Guatemala.