  We derive the expressions on the observed light-cone for some relevant
cosmological gauge invariant variables, such as the Mukhanov-Sasaki variable
and $E$- and $B$- modes of the tensor perturbations. Since the structure of the
light-cone does not reflect in a direct way the FLRW symmetries, we develop a
formalism which is coordinate independent and classifies the perturbations
according to their helicities. Even though we work with linear perturbations,
our formalism can be readily extended to non-linear theory and put the basis to
study the evolution of cosmological perturbations, since the early- until the
the late- time Universe, directly along the observed light-cone.
