  It is shown that in the absence of spontaneous symmetry breaking the Dirac
cones in the system of interacting electrons on honeycomb lattice are isotropic
at low energies. The effect is due to the $Z_3$ subgroup of the $D_3$ symmetry
group of the dispersion relation of Dirac quasiparticles. Consequences of the
violations of the $Z_3$ or the sublattice symmetry are discussed.
