  The exclusive photoproduction of $e^+e^-$ pairs from nucleons close to the
vector meson production threshold ($1.4<\sqrt s <1.8$ GeV) results from two
main processes: the emission of Bethe-Heitler pairs and the photoproduction of
$\rho^0$- and $\omega$-mesons decaying into $e^+e^-$ pairs. The Bethe-Heitler
amplitudes are purely electromagnetic and reflect mostly the nucleon magnetic
structure. The $\gamma N\to e^+e^- N$ amplitudes arising from vector meson
production and decay are derived from $\gamma N\to \rho^0 N$ and $\gamma N \to
\omega N$ amplitudes supplemented by the Vector Meson Dominance assumption. The
vector meson photoproduction amplitudes are calculated using a relativistic and
unitary coupled-channel approach to meson-nucleon scattering. They depend
sensitively on the coupling of vector fields to baryon resonances. The $\gamma
N \to e^+e^- N$ differential cross sections display interference patterns. The
interference of Bethe-Heitler pair production with vector meson $e^+e^-$ decay
is quite small in the domain of validity of our model for all angles of the
emitted $e^+e^-$ pair. The interference of $\rho^0$- and $\omega$-mesons in the
$e^+e^-$ channel can be large. It is constructive for the $\gamma p \to e^+e^-
p$ reaction and destructive for the $\gamma n \to e^+e^- n$ reaction. We
discuss the shape and magnitude of the $e^+e^-$ pair spectra produced in the
$\gamma p \to e^+e^- p$ and $\gamma n \to e^+e^- n$ reactions as functions of
the pair emission angle and of the total center of mass energy $\sqrt s$.
