  The formation of the photo-polaronic excitons in ABO_{3} perovskite type
oxides has been detected experimentally by means of the photoinduced electron
paramagnetic resonance studies of KTa_{0.998}Nb_{0.012}O_{3} crystals. The
corresponding microwave X-band spectrum at T < 10 K consists of a narrow,
nearly isotropic signal located at g ~ 2 and a strongly anisotropic component.
The first signal, which has a rich structure due to hyperfine interactions with
the lattice nuclei, is attributed to the single trapped charge carriers: the
electrons and/or the holes. The anisotropic spectrum is caused by the axial
centers oriented along the C_{4} pseudo-cubic principal crystalline axes. The
spectrum angular dependence can be described well by an axial center with S =
1, g_{\parallel) = 0.82, g_{\perp} = 0.52 and D = 0.44 cm^{-1}. The anisotropic
spectrum is attributed to the Nb^{4+}-O^{-} polaronic excitons. The temperature
dependence of the anisotropic component is characterized by two activation
energies: the internal dynamics activation E_{a1} = 3.7\pm0.5 meV, which makes
the EPR spectrum unobservable above 10 K, and the destruction energy E_{a2} =
52\pm4 meV. By comparing the anisotropic photo-EPR spectrum and the
photoinduced optical absorption temperature dependencies, we found that the
Nb^{4+}-O^{-} polaronic excitons also manifested themselves via the ~0.7 eV
wide absorption band arising under UV light excitation in the weakly
concentrated KTaO_{3}:Nb crystals.
