Born: April 25, 1945. Björn Kristian Ulvaeus is a Swedish singer-songwriter, producer, a member of the Swedish musical group ABBA, and co-composer of the musicals Chess, Kristina från Duvemåla, and Mamma Mia!. He co-produced the films Mamma Mia! and Mamma Mia! Here We Go Again with fellow ABBA member and close friend Benny Andersson.