  We study the electromagnetic wave propagation in the joint dilaton field and
axion field. Dilaton field induces amplification/attenuation in the propagation
while axion field induces polarization rotation. The amplification/attenuation
induced by dilaton is independent of the frequency (energy) and the
polarization of electromagnetic waves (photons). From observations, the
agreement with and the precise calibration of the cosmic microwave background
(CMB) to blackbody radiation constrains the fractional change of dilaton
|{\Delta}{\psi}|/{\psi} to less than about 8 x 10^(-4) since the time of the
last scattering surface of the CMB.
