Eta Carinae
Rotation rates of massive stars have a critical influence on their evolution and eventual death. The rotation rate of the Eta Carinae stars cannot be measured directly because their surfaces cannot be seen. Single massive stars spin down quickly due to braking from their strong winds, but there are hints that both Eta Carinae A and B are fast rotators, up to 90% of critical velocity. One or both could have been spun up by binary interaction, for example accretion onto the secondary and orbital dragging on the primary.