A cam phaser apparatus comprising a triple member gear system for transferring a driving torque from a drive member to a driven member and for selectively adjusting the angular relationship between the drive and driven members, said triple member gear system comprising a first member comprising a driving part, a second member comprising a driven part connectable to a cam sprocket or pulley, and a third member comprising an adjusting means connectable to a camshaft, said third member being connected to means for rotating said third member or applying a braking torque and/or a motoring torque to said third member to vary the angular relationship between the first and second members, wherein the cam phaser apparatus further comprises a selectively actuable coupling means for selectively coupling two of said first, second and third members. In one embodiment the coupling means comprises a magnetic coupling.