Non-Abelian gauge field dynamics on matrix D-branes in curved space and two-dimensional $\sigma$-models

A survey is given of the formulation of a $\sigma$-model describing an open string moving in general target space background fields and coupling to both a matrix-valued D-brane position and a matrix-valued gauge field on the D-brane. The equations of motion for the D-brane and the gauge field are derived from the conformal invariance condition on the string world sheet in lowest order of $\ap$. The ordering problem of the involved matrices is solved. In addition to our previous work we discuss a conflict between the classical T-duality rules and renormalization. The calculation of the RG $\beta$-functions does not yield the mass term obtained by formal application of these rules in the case of target space separated D-brane copies.