  We reanalyse the gravitational couplings of the perturbative orientifold
planes $Op^-$, $Op^+$ (and D-branes). We first compute their $D_{-1}$
instantonic corrections for $p=3$. Then, by using U-dualities, we obtain the
Wess-Zumino terms of orientifolds with RR flux for $p \leq 5$. The expressions
for the effective actions can be partially checked via M-theory. We point out a
previous oversimplification and we show in fact that the difficulty still
stands in the way of the full computation of 7 Brane instanton corrections.
