The $\mathbb{Z}_2$-orbifold/higher orientifold fixed point manifold in Hořava-Witten theory might be called it O9-plane but is often called the M9-brane (e.g. Moore-Peradze-Saulina 04), even though it is on a different conceptual footing than the genuine M2-brane and M5-brane.
In (Hull 97, pages 8-9) the M9-brane was argued to be the object whose charge is the Poincaré dual to the time-component of the M2-brane charge as it appears in the M-theory super Lie algebra via
(In the same way the time component of the M5-brane charge is argued to be dual to the charge of the KK-monopole, see there.)
Also, all D-branes of type II string theory can be understood as arising from known structures in M-theory – except for the “D8-brane” domain wall. Therefore there is a conjecture that apart from the M2-brane and the M5-brane there is also an M9-brane generally in M-theory. But the situation is inconclusive.
Table of branes appearing in supergravity/string theory (for classification see at brane scan).
Discussion of a possible black 9-brane in 11d is in
The term “M9-brane” maybe originates in
A proposal for a description of the M9 as an higher WZW theory is in
Discussion of how the M2-brane and the M5-brane may arise from this by tachyon condensation is in
Cohomological discussion of ninebrane structures is in
See also
Paul Howe, A. Kaya, Ergin Sezgin, P. Sundell, Codimension One Branes (arXiv:hep-th/0001169)
Gregory Moore, Grigor Peradze, Natalia Saulina, Instabilities in heterotic M-theory induced by open membrane instantons, Nucl.Phys. B607 (2001) 117-154 (arXiv:hep-th/0012104)
Takeshi Sato, On M-9-branes and their dimensional reductions (arXiv:hep-th/0102084)
Discussion of open M5-branes ending on the M9 is in