  We have identified the pieces of the Fermi surface responsible for the long
period oscillations of magnetic coupling across Cr and Cr$_{(1-x)}$V$_x$ alloy
spacers in metallic multilayers. Analysing experiments and results of KKR-CPA
calculations we find that the periods are determined by the extremal
wave~vectors of the hole pockets centered on the N-point in the Brillouin zone.
