  Fractional D6-branes on toroidal orbifold backgrounds are known to be able to
accommodate the particle spectrum and gauge group of the Standard Model, but up
to now exact results for their low-energy effective action are missing. Here we
discuss how the conceptual ansatz of matching the string theoretic gauge
couplings at one-loop with the supergravity expressions is generalised from the
six-torus to orbifold backgrounds on which the Standard Model spectrum can be
realised on fractional D6-branes. The Kaehler metrics and perturbatively exact
holomorphic gauge kinetic functions can be classified in terms of the vanishing
of some intersection angle and the related beta function coefficients, which
potentially opens the possibility to extrapolate to smooth Calabi-Yau
backgrounds.
