  We address the question of whether boson mediators of high-$T_c$
superconductivity are composite (electronic) or independent phonons. For s-wave
superconductivity we show from the available experiments that the hypothesis of
composite bosons is rather unlikely. Our analysis points naturally towards
phonon mediators. In addition we point out that the eventual presence of a peak
in the temperature dependence of the microwave conductivity while the
Hebel-Slichter peak is absent in the temperature dependence of the NMR
relaxation rate, can be understood within a phonon mechanism if one takes into
account the modulation of the electron-phonon coupling (predominance of forward
scattering) induced by Coulomb correlation of the carriers.
