  We investigate the Kondo model with time-dependent couplings that are
periodically switched on and off. On the Toulouse line we derive exact
analytical results for the spin dynamics in the steady state that builds up
after an infinite number of switching periods. Remarkably, the algebraic long
time behavior of the spin-spin correlation function remains completely
unaffected by the driving. In the limit of slow driving the dynamics become
equivalent to that of a single interaction quench. In the limit of fast driving
one can show that the steady state cannot be described by some effective
equilibrium Hamiltonian since a naive implementation of the Trotter formula
gives wrong results. As a consequence, the steady state in the limit of fast
switching serves as an example for the emergence of new quantum states not
accessible in equilibrium.
