  A magnetic Bragg reflection corresponding to the wave vector k13 =
(2pi/a)[1/2,1/2,1/2] of the antiferro-quadrupolar ordering is found in CeB6 in
zero magnetic field below the Neel temperature TN. Its intensity is two orders
of magnitude weaker than those due to the basic magnetic structure [O. Zaharko
et al., Phys. Rev. B 68, 214401 (2003)]. The peak has a width of the other
Bragg reflections below TN, but widens abruptly at T = TN with simultaneous
increase of intensity. Correlation length just above TN is of the order of 70
A. The peak intensity decreases to zero at T = 7 K with no visible anomaly at
the antiferro-quadrupolar ordering temperature TQ = 3.3 K. The features of this
magnetic ordering are typical for the itinerant magnetism with 5d electron of
Ce3+ [Yu.S. Grushko et al., phys. stat. sol. (b) 128, 591 (1985)] being
involved.
