We consider control systems of the type $\dot x = A x +α(t)bu$, where $u\in\R$, $(A,b)$ is a controllable pair and $α$ is an unknown time-varying signal with values in $[0,1]$ satisfying a persistent excitation condition i.e., $\int_t^{t+T}\al(s)ds\geq μ$ for every $t\geq 0$, with $0