  We present an analytical model that characterizes two-photon transitions in
the presence of autoionising states. We applied this model to interpret
resonant RABITT spectra, and show that, as a harmonic traverses a resonance,
the phase of the sideband beating significantly varies with photon energy. This
phase variation is generally very different from the $\pi$ jump observed in
previous works, in which the direct path contribution was negligible. We
illustrate the possible phase profiles arising in resonant two-photon
transitions with an intuitive geometrical representation.
