  The theory of linear stochastic thermodynamics is developed for periodically
driven systems in contact with a single reservoir. Appropriate thermodynamic
forces and fluxes are identified, starting from the entropy production for a
Markov process. Onsager coefficients are evaluated, the Onsager-Casimir
relations are verified, and explicit expressions are given for an expansion in
terms of Fourier components. The results are illustrated on a periodically
modulated two level system including the optimization of the power output.
