  Differential cross sections of the reactions $pp \to d\pi^+$ and $pp \to
pn\pi^+$ have been measured at $T_p = 400$ MeV by detecting the charged
ejectiles in the angular range $4^0 \leq \Theta_{Lab} \leq 21^\circ$. The
deduced total cross sections agree well with those published previously for
neighbouring energies. The invariant mass spectra are observed to be strongly
affected by $\Delta$ production and $NN$ final-state interaction. The data are
well described by Monte Carlo simulations including both these effects. The
ratio of $pp \to pn\pi^+$ and $pp \to d\pi^+$ cross sections also compares
favourably to a recent theoretical prediction which suggests a dominance of
$np$-production in the relative $^3S_1$-state.
