EN
This paper is concerned with general conditions for convergence rates of nonparametric orthogonal series estimators of the regression function. The estimators are obtained by the least squares method on the basis of an observation sample $Y_i = f(X_i) + η_i$, i=1,...,n, where $X_i ∈ A ⊂ ℝ^d$ are independently chosen from a distribution with density ϱ ∈ L¹(A) and $η_i$ are zero mean stationary errors with long-range dependence. Convergence rates of the error $n^{-1} ∑_{i=1}^n (f(X_i)-f̂_N(X_i))²$ for the estimator $f̂_N(x) = ∑_{k=1}^N ĉ_k e_k(x)$, constructed using an orthonormal system $e_k$, k=1,2,..., in L²(A), are obtained.