Effective diophantine approximation on
Let be a real number and let be a positive integer. We define four exponents of Diophantine approximation, which complement the exponents and defined by Mahler and Koksma. We calculate their six values when and is a real number whose continued fraction expansion coincides with some Sturmian sequence of positive integers, up to the initial terms. In particular, we obtain the exact exponent of approximation to such a continued fraction by quadratic surds.