What lucky star can Washington audiences be placed under that has permitted two concert appearances here by Frederica von Stade in little more than a month?

Last night, at the Kennedy Center Concert Hall, she sang with the instrumentalists of the Chamber Music Society of Lincoln Center, a group of which she has become a member.

Von Stade couldn't subordinate herself to being just another deferential figure in such an ensemble even if she wanted to -- like the second viola in last night's Dvorak String Sextet, Op. 48. When a voice is like hers, with that incomparably easy freshness, it simply asserts itself -- without need of any of the poses assumed by the usual mezzo. But her decision to join the chamber music society, with its emphasis on performing with others, not beyond others, is yet another token of this singer's seriousness of purpose.

Her songs on last night's program were of a different cut from what she sang in early April at Georgetown University. Most prominently, there was Mahler -- four songs from the "Ru ckert Lieder." This is weightier music than von Stade was singing last month, especially last night's final song, "Ich bin der Welt abhanden gekommen" ("I have become lost to the world"), with its premonition of the overwhelming stillness on which would conclude the great song cycle to come, "Das Lied von der Erde." The rapt way that von Stade strung out the final phrases, "In meinem Lieben, in meinem Lieben, in meinem Lied" ("In my love, in my love, in my song"), was marvelous. And Lee Luvisi played the piano part with comparably total immersion.

Von Stade also sang, with her usual grasp of the French idiom, that sad tribute to lost love, "Chanson perpe'tuelle," by Ernest Chausson. This was in the version for piano and string quartet. It still sounded orchestral, just as in the alternate orchestral arrangement. No one sings this kind of music better than she.

The evening opened with a perfunctory version of Haydn's string trio, Op. 53, by James Buswell, Walter Tramoler and Nathaniel Rosen. It needed more bite. Then came Rosen and Luvisi in the cello arrangement of Stravinsky's "Suite Italienne," yet another version of excerpts from the ballet, "Pulcinella."

The evening ended with a bristling performance of the Dvorak, which is a stirring and much underrated work.