Abstract.We calculate the wavefunction and the energy eigenvalues of two non-relativistic interacting particles in an external field with spherical symmetry. The techniques used and the formalism developed are suitable for all problems of this kind. We give explicit formulae for the matrixelements. As an example we discuss the case of a light charged fermion-antifermion pair in the Coulomb field of a heavy nucleus, assuming a confining potential between the fermions. PACS: 03.65.Ge; 31.20.Di; 12.90.+b move in the field of the heavy one. This simplifies the problem compared with the dr# case. The developed techniques and formulae can be used for any problem of this kind. In a subsequent publication [8] we will specifically discuss the following scenario, which was inspired by attempts to explain the anomalous electron-positron coincidences measured in heavy-ion collisions at GSI: two hypothetical light particlesf + and f-, bound together by a confinement potential, move in the Coulomb field of a heavy ion. In Sect. 3.2 these calculations are sketched as an illustration for the wide area of application for our code.