American Ballet Theater dancers looked wonderful on the second night of the company's season at the Metropolitan Opera. The program was two New York premieres, a dazzling ``Black Swan'' pas de deux and ``Lilac Garden'' starring Carla Fracci. The Italian ballerina returned to American Ballet Theater as a guest to perform in an excerpt from ``Lilac Garden'' at Monday's gala opening and the entire Antony Tudor ballet on Tuesday night. The entire ballet, to Chausson's ``Poeme,'' was as achingly romantic as it is meant to be. The young bride-to-be, danced by Miss Fracci with subtlety and gentle beauty, is stoical but suffering because in love with a younger man. He was danced with romantic yearning by Kevin McKenzie. Ross Stretton as the Man She Must Marry was stiff with his bride-to-be but softer when he met An Episode in His Past in the lilac garden. She, danced by Martine Van Hamel, seemed a strong survivor but she was hurting, too. The evening began with ``Birthday Offering,'' which Frederick Ashton choreographed for the 25th anniversary of the British Royal Ballet. It gives important solos to seven ballerinas, showing each one's strength. Ballet Theater added it to its repertory last October in Houston. On Tuesday, the star ballerina was Susan Jaffe, partnered by Stretton. The others were Cheryl Yeager, Amanda McKerrow, Julie Kent, Leslie Browne, Alessandra Ferri and Christine Dunham. All looked good. Fernando Bujones, back this season as a guest, having been greatly missed, and Cynthia Harvey danced the ``Black Swan'' pas de deux. Both were in the highest, exciting form. Toward the end of the duet, Miss Harvey danced only 16 of the famous 32 fouettes, whipped-leg turns. The audience murmured, fearing she had felt a muscle tear, but she finished the duet flawlessly. The evening ended with the half-hour ``Brief Fling'' which Twyla Tharp choreographed for the company. This eclectic, quite enjoyable ballet had its premiere in February in San Francisco. In it, Cheryl Yeager and Julio Bocca, in Navy, do formal ballet dancing to ``I'm a Little Teapot'' played on amplified piano. Shelley Washington, loose-limbed as Miss Tharp was as a dancer, looked like an urchin being tossed around by Jamie Bishton, Kevin O'Day and Keith Roberts in mini-kilts. Claudia Alfieri, Isabella Padovani, Gil Boggs and Robert Wallace wore red plaid. During the 10 sections of ``Brief Fling'' there also was a jig and music which sounded like minimalism meeting a cement mixer. The music was by Michel Colombier. Clever costumes were by Isaac Mizrahi. Dennis Cleveland played the violin solo in ``Lilac Garden.'' The orchestra sounded thin, as though too small for the requirements of the music.