D-brane

A D-brane is a hypersurface on which open-string endpoints obey Neumann boundary conditions tangentially and Dirichlet boundary conditions transversely. Its massless open-string vector mode is a worldvolume gauge field.

For a D-brane in metric $G$, pulled-back B-field $B$ and worldvolume field strength $F$, tangential open-string coordinates obey the mixed condition $G_{ab}{\partial }_{\sigma }{X}^b+(B+2\pi {\alpha }^{\prime }F{)}_{ab}{\partial }_{\tau }{X}^b=0$. Only the gauge-invariant combination $B+2\pi {\alpha }^{\prime }F$ is physical.