  We study the Josephson junction mediated by the quantum spin Hall edge states
and show that electron-electron interactions lead to a dissipationless
fractional Josephson effect in the presence of time-reversal symmetry.
Surprisingly, the periodicity is $8\pi$, corresponding to a Josephson frequency
$eV/2\hbar$. We estimate the magnitude of interaction induced many-body level
splitting responsible for this effect and argue that it can be measured using
tunneling spectroscopy. For strong interactions we show that the Josephson
effect is associated with the weak tunneling of charge $e/2$ quasiparticles
between the superconductors. Our theory describes a fourfold ground state
degeneracy that is similar to that of coupled "fractional" Majorana modes, but
is protected by time reversal symmetry.
