  We consider stationary, axially and equatorially symmetric systems consisting
of a central rotating and charged degenerate black hole and surrounding matter.
We show that $a^2+Q^2=M^2$ always holds provided that a continuous sequence of
spacetimes can be identified, leading from the Kerr-Newman solution in
electrovacuum to the solution in question. The quantity $a=J/M$ is the black
hole's intrinsic angular momentum per unit mass, $Q$ its electric charge and
$M$ the well known black hole mass parameter introduced by Christodoulou and
Ruffini.
