Björn Kristian Ulvaeus (credited as Björn Ulvæus ; born 25 April 1945) is a Swedish songwriter, producer, a former member of the Swedish musical group ABBA (1972–1982), and co-composer of the musicals "Chess", "Kristina från Duvemåla", and "Mamma Mia!". He co-produced the movie "Mamma Mia!" with fellow ABBA member and close friend Benny Andersson.