The Emerson String Quartet performs in a free concert at 8 p.m. Sept. 29 at Rackham Auditorium. Pianist Wu Han, who is married to Emerson cellist David Finckel, joins the group for the second half of the program. Violist Lawrence Dutton is taking a four-month leave from the quartet to undergo rotator cuff surgery, providing the opportunity to hear the other members in smaller chamber repertoire. The concert is sponsored by the University Musical Society. The group is scheduled to perform selections by Mozart and Brahms.