  The non homogeneity of the charge distribution in a carbon nanotube leads to
the formation of an excitonic resonance, in a similar way to the one observed
in X-ray absorption in metals. As a result, a positive anomaly at low bias
appears in the tunnelling density of states. This effect depends on the
screening of the electron--electron interactions by metallic gates, and it
modifies the coupling of the nanotube to normal and superconducting electrodes.
