  We prove that for any $\alpha$-mixing stationnary process the hitting time of
any $n$-string $A_n$ converges, when suitably normalized, to an exponential
law. We identify the normalization constant $\lambda(A_n)$. A similar statement
holds also for the return time. To establish this result we prove two other
results of independent interest. First, we show a relation between the rescaled
hitting time and the rescaled return time, generalizing a theorem by Haydn,
Lacroix and Vaienti. Second, we show that for positive entropy systems, the
probability of observing any $n$-string in $n$ consecutive observations, goes
to zero as $n$ goes to infinity.
