  The prevalence of long-lasting oscillatory signals in the 2d
echo-spectroscopy of light-harvesting complexes has led to a search for
possible mechanisms. We investigate how two causes of oscillatory signals are
intertwined: (i) electronic coherences supporting delocalized wave-like motion,
and (ii) narrow bands in the vibronic spectral density. To disentangle the
vibronic and electronic contributions we introduce a time-windowed Fourier
transform of the signal amplitude. We find that 2d spectra can be dominated by
excitations of pathways which are absent in excitonic energy transport. This
leads to an underestimation of the life-time of electronic coherences by 2d
spectra.
