  We demonstrate that $\pi$-kinks exist in non-parametrically ac driven
sine-Gordon systems if the ac drive is sufficiently fast. It is found that, at
a critical value of the drive amplitude, there are two stable and two unstable
equilibria in the sine-Gordon phase. The pairwise symmetry of these equilibria
implies the existence of a one-parameter family of $\pi$-kink solutions in the
reduced system. In the dissipative case of the ac driven sine-Gordon systems,
corresponding to Josephson junctions, the velocity is selected by the balance
between the perturbations. The results are derived from a perturbation analysis
and verified by direct numerical simulations.
