We discuss some properties of the conjectured M-9-brane. We investigate both the worldvolume action as well as the target space solution. The worldvolume action is given by a gauged sigma model which, via dimensional reduction and duality, is shown to be related to the worldvolume actions of the branes of ten- dimensional superstring theory. The effective tension of the M-9-brane scales as (R_11)^3, where R_11 is the radius of an S^1-isometry direction. This isometry enables us to add a

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... ical constant to eleven-dimensional supergravity. The target space solution corresponding to the M-9-brane is a (wrapped) domain wall of massive eleven-dimensional supergravity. This solution breaks half of the bulk supersymmetry. We consider both single M-9-branes as well as a system of two M-9-branes. In both cases one can define regions in spacetime, separated by the domain walls, with zero cosmological constant. In these regions the limit R_11-> ∞ can be taken in which case the M-9-brane is unwrapped and a massless eleven-dimensional supergravity is obtained.