Optimal regularity in the optimal switching problem

In this article we study the optimal regularity for solutions to the following weakly coupled system with interconnected obstacles \begin{equation*} \begin{cases} \min (-\Delta u^1+f^1, u^1-u^2+\psi^1)=0 \\ \min (-\Delta u^2+f^2, u^2-u^1+\psi^2)=0, \end{cases} \end{equation*} arising in the optimal switching problem with two modes. We derive the optimal $C^{1,1}$-regularity for the minimal solution under the assumption that the zero loop set $\mathscr{L}:= \{ \psi^1+\psi^2=0\}$ is the closure of its interior. This result is optimal and we provide a counterexample showing that the $C^{1,1}$-regularity does not hold without the assumption $\mathscr{L} = \overline{ \mathscr{L}^0}$.