Björn Kristian Ulvaeus (Swedish pronunciation: [bjœːɳ ɵlˈveːɵs]; born 25 April 1945; credited as Björn Ulvæus) is a Swedish songwriter, composer, musician, writer, producer, a former member of the Swedish musical group ABBA (1972–82), and co-composer of the musicals Chess, Kristina från Duvemåla, and Mamma Mia!. He co-produced the film Mamma Mia! with fellow ex-ABBA member and close friend Benny Andersson.