  We analyze the structure of an $s-$wave superconducting gap in systems with
electron-phonon attraction and electron-electron repulsion. Earlier works have
found that superconductivity develops despite strong repulsion, but the gap,
$\Delta (\omega_m)$, necessarily changes sign along the Matsubara axis. We
analyze the sign-changing gap function from a topological perspective using the
knowledge that a nodal point of $\Delta (\omega_m)$ is the center of dynamical
vortex. We consider two models with different cutoffs for the repulsive
interaction and trace the vortex positions along the Matsubara axis and in the
upper frequency half plane upon changing the relative strength of the
attractive and repulsive parts of the interaction. We discuss how the presence
of dynamical vortices affects the gap structure along the real axis, detectable
in ARPES experiments.
