  We study parity symmetries and boundary conditions in the framework of gauged
linear sigma models. This allows us to investigate the Kaehler moduli
dependence of the physics of D-branes as well as orientifolds in a Calabi-Yau
compactification. We first determine the parity action on D-branes and define
the set of orientifold-invariant D-branes in the linear sigma model. Using
probe branes on top of orientifold planes, we derive a general formula for the
type (SO vs Sp) of orientifold planes. As applications, we show how
compactifications with and without vector structure arise naturally at
different real slices of the Kaehler moduli space of a Calabi-Yau
compactification. We observe that orientifold planes located at certain
components of the fixed point locus can change type when navigating through the
stringy regime.

High Energy Physics - Theory;Algebraic Geometry