  We report on an experiment showing that the wavelength of a biphoton is
clearly dependent on the measurement scheme and on the way it is defined. It is
shown that it can take any value, depending on the control of the
interferometer phase differences. It is possible to identify the interference
of the single and two-photon wavepackets as particular cases of the most
general interference process. The variable wavelength has no implication on the
energy of the individual photons neither on the total energy of the biphoton.
