  It is shown that production of low-mass $e^+e^-$-pairs in ultraperipheral
nuclear collisions is enhanced due to the Sommerfeld-Gamow-Sakharov (SGS)
factor. This effect is especially strong near the threshold of creation of
unbound $e^+e^-$-pairs with low masses in the two-photon fusion. Coulomb
attraction of the non-relativistic components of such pairs may lead to the
increased intensity of 511 keV photons. It can be recorded at the NICA collider
and has some astrophysical implications. The analogous effect can be observed
at LHC in dilepton production.
