  The minimum entropy output is computed for rotationally invariant quantum
channels acting on spin-1/2 and spin-1 systems. For the case of two parallel
such channels and initial entangled (singlet) state the entropy of the output
is higher then the doubled minimal entropy output of the single channel. This
gives a certain moral support to the additivity hypothesis. Another related
simple function of the channel (minimum entropy gain) is shown to be additive
in general.
