  We update the D3/D7-brane inflation model on K3 x T^2/Z_2 with branes and
fluxes. For this purpose, we study the low energy theory including g_s
corrections to the gaugino condensate superpotential that stabilizes the K3
volume modulus. The gauge kinetic function is verified to become holomorphic
when the original N=2 supersymmetry is spontaneously broken to N=1 by bulk
fluxes. From the underlying classical N=2 supergravity, the theory inherits a
shift symmetry which provides the inflaton with a naturally flat potential. We
analyze the fate of this shift symmetry after the inclusion of quantum
corrections. The field range of the inflaton is found to depend significantly
on the complex structure of the torus but is independent of its volume. This
allows for a large kinematical field range for the inflaton. Furthermore, we
show that the D3/D7 model may lead to a realization of the recent CMB fit by
Hindmarsh et al. with an 11% contribution from cosmic strings and a spectral
index close to n_s=1. On the other hand, by a slight change of the parameters
of the model one can strongly suppress the cosmic string contribution and
reduce the spectral index n_s to fit the WMAP5 data in the absence of cosmic
strings. We also demonstrate that the inclusion of quantum corrections allows
for a regime of eternal D3/D7 inflation.
