  We define the heat capacity for steady periodically driven systems and as an
example we compute it for dissipative two-level systems where the energy gap is
time-modulated. There, as a function of ambient temperature, the Schottky peak
remains the dominant feature. Yet, in contrast with equilibrium, the
quasistatic thermal response of a nonequilibrium system also reveals kinetic
information present in the transition rates; e.g., the heat capacity depends on
the time-symmetric reactivities and changes by the presence of a kinetic
barrier. It still vanishes though at absolute zero, in accord with an extended
Nernst heat postulate, but at a different rate from the equilibrium case. More
generally, we discuss the dependence on driving frequency and amplitude.
