  Starting from the orthogonal polynomial expansion of a function $F$
corresponding to a finite positive Borel measure with infinite compact support,
we study the asymptotic behavior of certain associated rational functions
(Pad\'{e}-orthogonal approximants). We obtain both direct and inverse results
relating the convergence of the poles of the approximants and the singularities
of $F.$ Thereby, we obtain analogues of the theorems of E. Fabry, R. de
Montessus de Ballore, V.I. Buslaev, and S.P. Suetin.
