  Two-particle backscattering in time-reversal invariant interacting helical
electron systems can lead to the formation of quasiparticles with charge $e/2$.
We propose a way to detect such states by means of the Josephson effect in the
presence of proximity-induced superconductivity. In this case, the existence of
$e/2$ charges leads to an $8 \pi$-periodic component of the Josephson current
which can be identified through measurement of Shapiro steps in Josephson
junctions. In particular, we show that even when there is weak explicit
time-reversal symmetry breaking, which causes the two-particle backscattering
to be a sub-leading effect at low energies, its presence can still be detected
in driven, current-biased Shapiro step measurements. The disappearance of some
of these steps as a function of the drive frequency is directly related to the
existence of non-Abelian zero-energy states. We suggest that this effect can be
measured in current state-of-the-art Rashba wires.
