  It is proven that the sum rules for X-ray magnetic dichroism (XMCD) spectra
that are used to separate spin and orbital contributions to the magnetic moment
are formally correct for an arbitrary strength of electron-electron
interactions. However, their practical application for strongly correlated
systems can become complicated due to the spectral density weight spreading
over a broad energy interval. Relevance of incoherent spectral density for the
XMCD sum rules is illustrated by a simple model of a ferromagnet with orbital
degrees of freedom.
