  Exceptional points as branch singularities describe peculiar degeneracies of
non-Hermitian systems that do not obey energy conservation. This work shows
that exceptional points can emerge in a topological photonic system, for
example, the topological photonic waveguide coupled with two degenerate
counter-propagation topological whispering gallery modes. Such a photonic
architecture is designed by crystal-symmetry-protected topological photonic
insulators based on air rods in conventional dielectric materials. The relevant
exceptional point reveals the breaking of the parity-time symmetry, reflected
by the change of the transmission-dip number in the optical transmission
spectra of the system. Achieving exceptional points in topological photonic
systems possibly opens a new avenue toward robust optical devices with
exceptional-point-based unique properties and functionalities.
