A D8-brane is a D-brane with eight spatial directions and a nine-dimensional worldvolume. In ten-dimensional type IIA superstring theory, it has one transverse spatial direction, so it acts as a domain wall. Its charge is associated with a nine-form Ramond–Ramond potential.
The dual zero-form field strength is the Romans mass; its value can jump across a charged D8-brane. Consequently a D8-brane cannot simply be inserted into the elementary massless M-theory circle duality dictionary used for lower-dimensional IIA branes. Its gravitational matching and scalar-field profiles must be treated consistently on both sides of the wall.[ref]
Non-BPS D8-branes and Dynamic Domain Walls in Massive IIA Supergravities by A. Chamblin and others. abstract and domain-wall formulation: D8 sources in Romans theory and matching of the two sides
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