The American female a cappella quartet Anonymous 4 singing the conductus O Maria O Felix Puerpera, an elegant monophonic chant featured on their 2002 album La Bele Marie - Songs to the Virgin from 13th-century France. The singers are (left to right) - Susan Hellauer, Jacqueline Horner-Kwiatek, Marsha Genensky, Johanna Maria Rose (Johanna was later replaced by Ruth Cunningham). The composer of this simple yet absolutely glorious piece is unknown.