  For the module category of a hereditary ring, the Ext-orthogonal pairs of
subcategories are studied. For each Ext-orthogonal pair that is generated by a
single module, a 5-term exact sequence is constructed. The pairs of finite type
are characterized and two consequences for the class of hereditary rings are
established: homological epimorphisms and universal localizations coincide, and
the telescope conjecture for the derived category holds true. However, we
present examples showing that neither of these two statements is true in
general for rings of global dimension 2.
