Stake in Gradco Raised: Plenum Publishing Co., a small New York publisher of technical journals, has raised its stake in Irvine-based Gradco Systems Inc. to 8.9%. In a filing with the Securities and Exchange Commission, Plenum said it plans to nominate one or more candidates for Gradco’s board of directors at the company’s annual meeting on Aug. 31. Last December, Plenum disclosed in an SEC filing that it had acquired 6.3% of Gradco and said then that it might try to take a controlling interest in the firm. In late December and January, Plenum boosted its stake to 8%. Plenum officials could not be reached for comment Monday.