  Additivity of the Holevo capacity is proved for product channels, under the
condition that one of the channels is in a certain class of unital qubit
channels, with the other completely arbitrary. This qubit class includes the
depolarizing channel. As a byproduct this proves that the Holevo bound is the
ultimate information capacity of such qubit channels (assuming no prior
entanglement between sender and receiver). Additivity of minimal entropy and
multiplicativity of p-norms are also proved under the same assumptions.
