  Five-branes lead in four dimensions to massless N=1 supermultiplets if
M-theory is compactified on S1/Z2 x (a Calabi-Yau threefold). One of them
describes the modulus associated with the position of the five-brane along the
circle S1. We derive the effective four-dimensional supergravity of this
multiplet and its coupling to bulk moduli and to Yang-Mills and charged matter
multiplets located on Z2 fixed planes. The dynamics of the five-brane modes is
obtained by reduction and supersymmetrization of the covariant five-brane
bosonic action. Our construction respects all symmetries of M-theory, including
the self-duality of the brane antisymmetric tensor. Corrections to gauge
couplings are strongly constrained by this self-duality property. The brane
contribution to the effective scalar potential is formally similar to a
renormalization of the dilaton. The vacuum structure is not modified.
Altogether, the impact of the five-brane modulus on the effective supergravity
is reminiscent of string one-loop corrections produced by standard
compactification moduli.
