  In an inflationary regime driven by a free massive inflaton we derive within
a genuinely gauge invariant approach the backreaction effects due to long
wavelength scalar fluctuations on the effective Hubble factor and equation of
state with respect to a class of observers which sees an inhomogeneous and
isotropic Universe. We find that, for such so-called isotropic observers,
contrary to what happens for the observables defined by free-falling observers,
there is an effect to leading order in the slow-roll parameter in the direction
of slowing down the measured rate of expansion and of having an effective
equation of state less de Sitter like. From a general point of view the
isotropic observers result has to be considered complementary to other cases
(observers) in helping to characterize the physical properties of the models
under investigation.
