We find the moment generating function (mgf) of the nonequilibrium work for open systems undergoing a thermal process, ie, when the stochastic dynamics maps thermal states into time dependent thermal states. The mgf is given in terms of a temperature-like scalar satisfying a first order ODE. We apply the result to some paradigmatic situations: a levitated nanoparticle in a breathing optical trap, a brownian particle in a box with a moving piston and a two state system driven by an external field, where the work mgfs are obtained for different timescales and compared with Monte Carlo simulations.

5 pages, 5 figures