  Electron fractionalization is intimately related to topology. In
one-dimensional systems, fractionally charged states exist at domain walls
between degenerate vacua. In two-dimensional systems, fractionalization exists
in quantum Hall fluids, where time-reversal symmetry is broken by a large
external magnetic field. Recently, there has been a tremendous effort in the
search for examples of fractionalization in two-dimensional systems with
time-reversal symmetry. In this letter, we show that fractionally charged
topological excitations exist on graphenelike structures, where quasiparticles
are described by two flavors of Dirac fermions and time-reversal symmetry is
respected. The topological zero-modes are mathematically similar to fractional
vortices in p-wave superconductors. They correspond to a twist in the phase in
the mass of the Dirac fermions, akin to cosmic strings in particle physics.
