By E. Graydon Carter

They didn't laugh when he sat down at the Steinway, but then for a change, they weren't supposed to. Last week Dudley Moore, 48, made his Carnegie Hall debut as a full-dress, classical pianist. Joining his friend, Violinist Robert Mann, 62, Cellist Nathaniel Rosen, 36, and the St. Paul Chamber Orchestra under the baton of Conductor-Violinist Pinchas Zukerman, 34, Moore offered a sensitive, well-paced performance of Beethoven's Triple Concerto. And he played it straight, until the very end. Then, just before heading offstage for a congratulatory hug from his longtime squeeze, Susan Anton, 32, he reverted to the depths of British music hall humor. Turning his back to the audience, Moore impishly flipped the tails of his jacket in the air as he bade Carnegie Hall adieu.

Friendships do not always grow well in the glittery sand of show biz. But Actresses Nastassia Kinski, 22, and Jodie Foster, 20, have become chums despite the glare. After meeting at an Aretha Franklin concert a year and a half ago, Occasional Journalist Foster, a junior at Yale, took up recorder and notebook for a Q. and A. dialogue with Kinski that ran, seemingly forever, in Interview magazine. They are reunited for the film adaptation of John Irving's Hotel New Hampshire, and the friendship continues. Luckily. The script calls for Kinski--who in the film spends the better part of two reels cooped up in a bear costume--to share a love scene with Foster in which they briefly kiss. "It seemed like an extension of our deep friendship," says Nastassia, "although we both joked about it later."

Some clues: the culprit (or lucky fellow, depending on how one looks at it) was a member of Britain's '70s Labor government and is still an influential M.P. He is married, has a daughter and is described as being like a "bear," with a shock of dark, tumbling hair. Finally, "Gerry" as he is referred to, is missing the tip of the little finger of his left hand. Such are the telltale hints dropped by Actress Shirley MacLaine, 49, in her recent autobiography Out on a Limb, describing a gent she once had an affair with. Her enigmatic references to Gerry have kicked off, in the land of Sherlock Holmes and Miss Marple, a full-blown, real-life mystery. Labor Leader Michael Foot, 69. denied any role in the affair. So did David Owen, 44, Tony Benn, 58, and Peter Shore, 59, who last week all raised their pinkies in emphatic, if somewhat wistful, denial of ever having had anything to do with MacLaine. Then she allowed that perhaps she did some Gerrymandering of the clues to conceal her ex-lover's identity. So still high on the list of suspects is Andrew Peacock, 44, a dashing member of Australia's last Conservative government, who is a longstanding friend of the actress. MacLaine now considers all the fuss a bother. "Affairs of state," she told a pack of sleuthing British reporters, "are more important than the state of my affairs."

In France, where tennis champions have been as scarce as courteous taxi drivers, Yannick Noah, 23, has become the most talked about addition to center court since Rene Lacoste first stitched a crocodile onto a cotton shirt. In the finals of the French Open, Noah defeated Swedish Ace Mats Wilander, 18, the defending champion, 6-2, 7-5, 7-6. Delirious fans overflowed onto the court as he became the first Frenchman to win the tournament since 1946. (Noah was born in France and raised in the former French African territory of Cameroon.) Dampening Yannick's win only slightly was a 42-day suspension and $20,000 fine from the International Tennis Federation for his refusal to participate in a scheduled tournament last month. The punishment was stiff, but nowhere near as harsh as the one levied last week against Argentine Gulilermo Vilas, 31, who drew a year's suspension for accepting money simply to show up for a tournament in Rotterdam.

--By E. Graydon Carter This file is automatically generated by a robot program, so viewer discretion is required.