  Let $B$ be a bialgebra, and $A$ a left $B$-comodule algebra in a braided
monoidal category $\Cc$, and assume that $A$ is also a coalgebra, with a
not-necessarily associative or unital left $B$-action. Then we can define a
right $A$-action on the tensor product of two relative Hopf modules, and this
defines a monoidal structure on the category of relative Hopf modules if and
only if $A$ is a bialgebra in the category of left Yetter-Drinfeld modules over
$B$. Some examples are given.
