  Exchange vector currents are calculated up to one-loop order (corresponding
to next-to-next-to-leading order) in chiral perturbation theory. As an
illustration of the power of the approach, we apply the formalism to the
classic nuclear process $n+p\rightarrow d +\gamma$ at thermal energy. The
exchange current correction comes out to be $(4.5 \pm 0.3)$ \% in amplitude
giving a predicted cross section $\sigma= (334\pm 3)\ {\mbox mb}$ in excellent
agreement with the experimental value $(334.2\pm 0.5)\ {\mbox mb}$. Together
with the axial charge transitions computed previously, this result provides a
strong support for the power of chiral Lagrangians in nuclear physics. As a
by-product of our results, we suggest an open problem in the application of
chiral Lagrangian approach to nuclear processes that has to do with giving a
physical meaning to the short-range correlations that play an important role in
nuclei.
