  A method is described for the development of the one-loop effective action
expansion as an asymptotic series in inverse powers of the fermion mass. The
method is based on the Schwinger-DeWitt proper-time technique, which allows for
loop particles with non-degenerate masses. The case with SU(2)xSU(2) as the
symmetry group is considered. The obtained novel series generalizes the
well-known Schwinger-De Witt inverse mass expansion for equal masses, and is
chiral invariant at each order. We calculate the asymptotic coefficients up to
fifth order and clarify their relationship with the standard Seeley-DeWitt
coefficients.
