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18 pages, 15 figures, 10 tables.-- PACS number(s): 75.50.Bb, 75.50.Ww, 76.80.+y ; The paper presents an iron-57 Mössbauer spectral study of RFe11.5Ta0.5, with R=Tb, Dy, Ho, Er, Lu, and an evaluation of the different contributions to the hyperfine magnetic fields. The Mössbauer spectra have been analyzed with a model that considers both the distribution of the tantalum atoms in the near-neighbor environment of the iron atoms and the relative orientation of the hyperfine field and the principal axis of the electric field gradient. Their possible directions in the ThMn12 structure have been determined from a close examination of the point symmetry of each iron site. A local model for the hyperfine field which enables to determine their components from experimental data, has been developed and a calculation of the lattice dipolar hyperfine field in RFe11.5Ta0.5 has been performed. We have investigated in detail the origin and influence of the contributions to the hyperfine field coming from self 3d polarization, the iron and rare earth transferred fields and the orbital and dipolar hyperfine fields. The iron and rare earth transferred fields have been analyzed for RFe11.5Ta0.5 and other rare earth-iron intermetallic compounds. From this analysis it is shown that the iron transferred fields are different at each crystallographic site, and comparable to the self 3d polarization contributions, and that the rare earth transferred field is mainly originated by the indirect exchange between the rare earth 4f and iron 3d electrons. ; This work was financed by the MAT 02/166 and MAT 2000/0107/P4-02 MCYT projects (Spanish government). C.P. thanks MCYT for financial Grant No. PN73195091Y. ; Peer reviewed