  We develop a theory to calculate the effective phase diffusion coefficient
and the mean phase velocity in periodically driven stochastic models with two
discrete states. This theory is applied to a dichotomically driven Markovian
two state system. Explicit expressions for the mean phase velocity, the
effective phase diffusion coefficient and the P\'eclet number are analytically
calculated. The latter shows as a measure of phase-coherence forced
synchronization of the stochastic system with respect to the periodic driving.
In a second step the theory is applied to a non Markovian two state model
modeling excitable systems. The results prove again stochastic synchronization
to the periodic driving and are in good agreement with simulations of a
stochastic FitzHugh-Nagumo system.
