The third and last show of the York Theatre Company's "Musicals in Mufti" season, Billion Dollar Baby, will run Sept. 25-27. Debbie Gravitte (Jerome Robbins' Broadway), Richard B. Schull, Marc Kudisch, Michael McCormick, Kristin Chenoweth (A New Brain), Lesley Blumenthal, James Darrah, Susan Owen, Charles Pistone, Darcie Roberts, Casey Nickolaw and Kay Walbye will appear.

Billion, of 1945, was Betty Comden and Adolph Green's first project after On the Town. Morton Gould wrote the music. The musical follows the rise of a 1920s golddigger who climbs to the top only to get a shock at the end of the show. B.T. McNicholl directs. All the concerts are performed at York's home in The Theatre at Saint Peter's Church in the Citicorp Center (Lexington Ave. and 54th St.). Audience discussions follow matinee performances. For information, call (212) 935-5820.

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The first show of the York's mainstage theatre season will most probably be the new, sung-through musical Little By Little, by Brad Ross (music), Hal Hackady & Ellen Greenfield (lyrics), and Greenfield and Annette Jolles (story). Cy Coleman's Exactly Like You will follow.

The season's third show hasn't been chosen yet. Candidates include Herringbone (by Tom Cone, Ellen Fitzhugh & Skip Kennon), the NY premiere of After The Fair, and the aforementioned Darling of the Day.