  We use a relationship between response and correlation function in
nonequilibrium systems to establish a connection between the heat production
and the deviations from the equilibrium fluctuation-dissipation theorem. This
scheme extends the Harada-Sasa formulation [Phys. Rev. Lett. 95, 130602
(2005)], obtained for Langevin equations in steady states, as it also holds for
transient regimes and for discrete jump processes involving small entropic
changes. Moreover, a general formulation includes two times and the new
concepts of two-time work, kinetic energy, and of a two-time heat exchange that
can be related to a nonequilibrium "effective temperature". Numerical
simulations of a chain of anharmonic oscillators and of a model for a molecular
motor driven by ATP hydrolysis illustrate these points.
