A neutron diffraction study of the low-temperature magnetic phase of

The low-field, low-temperature magnetic phase of the tetragonal compound $\rm DyRu_2Si_2$ is studied by neutron diffraction on a single crystal. The analysis of all the harmonics of the fundamental long-period commensurate propagation vector is consistent with the ($40\bar 4$) magnetic structure in which one ninth of the moments of the magnetic unit cell vanish. The origin of such a non-usual structure is discussed