The geometry of fractional branes

By looking at fractional Dp-branes of type IIA on T-4/Z(2) as wrapped brane s and by using boundary state techniques we construct the effective low-ene rgy action for the fields generated by fractional branes, build their world volume action and find the corresponding classical geometry. The explicit f orm of the classical background is consistent only outside an enhancon sphe re of radius r(e), which encloses a naked singularity of repulson-type, The perturbative running of the gauge coupling constant, dictated by the NS-NS twisted field that keeps its one-loop expression at any distance, also fai ls at r(e). (C) 2001 Elsevier Science B,V, All rights reserved.