WASHINGTON, May 28: A federal judge formally ordered two presidential aides to tell a grand jury about their discussions with President Bill Clinton and his wife concerning Monica Lewinsky, calling it ``some of the most relevant and important evidence'' in the investigation, according to released court documents.The documents also divulged that among the issues presidential aides previously refused to testify about before a grand jury were ``possible impeachment proceedings'' and Clinton confidant Bruce Lindsey's efforts to interview grand jury witnesses.In rejecting the President's efforts to block testimony with an executive privilege claim and a separate claim of attorney-client privilege, US district judge Norma Holloway Johnson did give the White House a partial victory.The judge concluded that a President's conversations with aides about such a matter as an investigation of private conduct can be covered by executive privilege, including discussions involving Hillary Rodham Clinton.``ThePresident does need to address personal matters in the context of his official decisions,'' the judge wrote. But Johnson ruled that even if the conversations are covered by executive privilege the right of a President to get confidential advice Whitewater prosecutor Kenneth Starr had proven that his need for the information was greater. She ordered that Lindsey and another presidential aide, Sidney Blumenthal, answer questions they previously refused to answer on grounds of executive privilege.Prosecutors have provided a substantial factual showing to demonstrate its ``specific need for the testimony,'' Johnson ruled in an order signed on Tuesday but which lawyers had been told of more than a week ago. Sources had confirmed the ruling more than a week ago, but the edited court documents formally released yesterday gave the first glimpse of the judge's reasoning.In her ruling, Johnson concluded that Starr was entitled to the evidence in part because it ``cannot feasibly be obtained elswhere.'' Andshe addressed the serious nature of what prosecutors are investigating whether Clinton encouraged Lewinsky to lie about an alleged presidential affair in the Paula Jones' sexual harassment lawsuit.Blumenthal immediately issued a statement saying he would testify. ``If called before the grand jury, I will testify truthfully and completely to all questions posed to me by Ken Starr,'' he said.White House counsel Charles Ruff said while the judge ultimately ordered the aides to testify she did affirm the White House arguments that the disputed conversations.President unwillingUS President Bill Clinton has rebuffed prosecutors' requests to voluntarily testify in the Monica Lewinsky investigation.Four months ago Clinton vowed to present the facts about his relationship with Lewinsky, saying then it was not sexual. But since February, however, he has declined as many as four or five invitations from Whitewater prosecutors to testify in the investigation.Clinton's lawyers have given variousexplanations in rejecting requests for him to give testimony, ranging from a busy schedule to White House distrust of prosecutors, said sources familiar with the dispute, speaking only on condition of anonymity.