  General--relativistic, frequency--dependent radiative transfer in spherical,
differentially--moving media is considered. In particular we investigate the
character of the differential operator defined by the first two moment
equations in the stationary case. We prove that the moment equations form a
hyperbolic system when the logarithmic velocity gradient is positive, provided
that a reasonable condition on the Eddington factors is met. The operator,
however, may become elliptic in accretion flows and, in general, when gravity
is taken into account. Finally we show that, in an optically thick medium, one
of the characteristics becomes infinite when the flow velocity equals $\pm
c/\sqrt 3$. Both high--speed, stationary inflows and outflows may therefore
contain regions which are ``causally'' disconnected.
