  We describe a peculiar fine structure acquired by the in-plane optical phonon
at the Gamma-point in graphene when it is brought into resonance with one of
the inter-Landau-level transitions in this material. The effect is most
pronounced when this lattice mode (associated with the G-band in graphene Raman
spectrum) is in resonance with inter-Landau-level transitions 0 -> (+,1) and
(-,1) -> 0, at a magnetic field B_0 ~ 30 T. It can be used to measure the
strength of the electron-phonon coupling directly, and its filling-factor
dependence can be used experimentally to detect circularly polarized lattice
modes.
