On the meaning of the separation constant in the Kerr metric

The existence of a fourth constant of motion, beyond rest mass, energy and axial angular momentum, for a free particle in a Ken spacetime has been sho wn by Carter through the separability of the Hamilton-Jacobi equation using oblate spheroidal coordinates. This fourth constant of motion is connected to a second-rank Stackel-Killing tensor related to the symmetries of the K en solution; while this mathematical aspect is clear, the physical meaning of the separation constant is not. In this note we solve this problem, show ing how the separation constant can be interpreted physically.