  We report on the investigation of dielectron production in tagged quasi-free
neutron-proton collisions by using a deuteron beam of kinetic energy 1.25 GeV/u
inpinging on a liquid hydrogen target. Our measurements with HADES confirm a
significant excess of $e^+e^-$ pairs above the $\pi^{0}$ mass in the exclusive
channel $dp \to npe^{+}e^{-}(p_{spect})$ as compared to the exclusive channel
$ppe^{+}e^{-}$ measured in proton-proton collisions at the same energy. That
excess points to different bremsstrahlung production mechanisms. Two models
were evaluated for the role of the charged pion exchange between nucleons and
double-$\Delta$ excitation combined with intermediate $\rho$-meson production.
Differential cross sections as a function of the $e^+e^-$ invariant mass and of
the angles of the virtual photon, proton and electrons provide valuable
constraints and encourage further investigations on both experimental and
theoretical side.
