Celestine BOHLEN, a mix of restitution and collecting. Ronald S- Lauder, heir to the Estée Lauder cosmetics fortune, former American ambassador to Austria, one-time candidate for mayor of New York, prodigious art collector and major benefactor of Jewish causes, knows a lot about art stolen by the Nazis, much of it from Jews. Starting in the mid-1990's, he became a vocal champion of restitution of the artworks to their rightful heirs, an issue that was then erupting across Europe and the United States after 50 years of silence. As chairman of the Commission for Art Recovery of the World Jewish Congress, Mr- Lauder has been a patron of scattered efforts to help Jews reclaim what had been theirs. In testimony before Congress, he called these stolen artworks "the last prisoners of war". But in an interview he also conceded that he had artworks in his collection whose provenance was at best ambiguous and at worst unknowable. And some critics say he has been too slow to check the provenance of his art, even given the historical difficulties of doing it. Mr- Lauder has a particular interest for two turn-of-the century Austrian artists, Egon Schiele and Gustav Klimt, many of whose works belonged to Jewish collectors before World War II. Many were stolen and lost during the Nazi years, and many of their owners were killed in the Holocaust. Could it be that some of the missing drawings are hanging on the walls of the Neue Galerie? The answer, Mr- Lauder and experts say, may be unknowable. "As for me, I am going to doubly, triply and quadruply check everything," Mr- Lauder said. "But that doesn't mean it couldn't happen". He and his curators have since done provenance research on the works in the museums collection. Still, the Neue Galerie has yet to post the results of its provenance research on its Web site in accordance with a commitment two years ago by the American Association of Museums to the Presidential Advisory Commission on Holocaust Assets. The agreement, covering art acquired after 1932 and produced before 1946, was intended to allow people anywhere a chance to look at American museum collections without having to travel here. Because of his prominent position in the New York art world - he is chairman of the Museum of Modern Art, as well as a founder of the Neue Galerie and one of the city's best known collectors - experts in the field of art stolen by the Nazis are reluctant to comment publicly on his record. But several, speaking anonymously, noted that his different, sometimes overlapping roles have sometimes clashed awkwardly. The issue of Holocaust-era art became news in New York in 1998 when District Attorney Robert M- Morgenthau moved to seize two Schiele paintings that had been on exhibit at the Museum of Modern Art, on loan from the Leopold Foundation in Vienna. The works, "Portrait of Wally" and "Dead City III," were claimed by the relatives of their original owners, Viennese Jews whose collections and property were seized by the Nazis. "Dead City III" was eventually sent back to Vienna, although some experts continue to challenge its provenance, which is similar to the provenance of Schiele drawings in some American museums, including a drawing entitled "Prostitute" at the Modern and another drawing, "I Love Àntithesis," at the Neue Galerie. As the Modern's chairman, Mr- Lauder implicitly agreed with the museum's legal position on the seized paintings, which supported the Leopold Foundation's arguments that American courts do not have the right to intervene in the affairs of another country. But four years later, M- Lauder, as chairman of the Commission for Art Recovery, protested when the State Department, using the same argument, topped a California court case in which an American heir was suing Austria for the return of six Klimt paintings taken from her uncle by the Nazis. Asked about the inconsistency, Mr- Lauder said he excluded himself from participating in the Modern's case "because of wearing two or three hats".