  We construct the most general chirally-invariant Lagrangian for mesons in the
presence of external sources coupled to the tensor current
\bar{\psi}\sigma_{\mu\nu}\psi. In order to have only even terms in the chiral
expansion, we consider the new source of O(p^2). With this choice, we build the
even-parity effective Lagrangian up to the p^6-order (NLO). While there are
only 4 new terms at the p^4-order, at p^6-order we find 78 terms for n_f=2 and
113 terms for n_f=3. We provide a detailed discussion on the different
mechanisms that ensure that our final set of operators is complete and
non-redundant. We also examine the odd-parity sector, to conclude that the
first operators appear at the p^8-order (NNLO).
