  The spectral weight (SW) for optical transitions in FeSi and MnSi are
calculated as function of temperature by means of LMTO-LDA band calculations.
The main effects, caused by structural disorder and electronic Fermi-Dirac
distribution, act oppositely on the T-dependence of the SW, while the variation
of the magnetic moment in MnSi has only a minor effect. The calculations agree
with the experimental findings of an increasing SW in FeSi and a decreasing SW
in MnSi as function of T. The results can be understood from the change of the
bandstructure with disorder.
