  This dissertation reviews some properties of the low-energy effective actions
for six dimensional open-string models. The first chapter is a pedagogical
introduction about supergravity theories. In the second chapter closed strings
are analyzed, with particular emphasis on type IIB, whose orientifold
projection, in order to build type-I models, is the subject of the third
chapter. Original results are reported in chapters 4 and 5. In chapter 4 we
describe the complete coupling of (1,0) six-dimensional supergravity to tensor,
vector and hypermultiplets. The generalized Green-Schwarz mechanism implies
that the resulting theory embodies factorized gauge and supersymmetry
anomalies, to be disposed of by fermion loops. Consequently, the low-energy
theory is determined by the Wess-Zumino consistency conditions, rather than by
the requirement of supersymmetry, and this procedure does not fix a quartic
coupling for the gauginos. In chapter 5 we describe the low-energy effective
actions for type-I models with brane supersymmetry breaking, resulting form the
simultaneous presence of supersymmetric bulks, with one or more gravitinos, and
non-supersymmetric combinations of BPS branes.The consistency of the resulting
gravitino couplings implies that local supersymmetry is non-linearly realized
on some branes. We analyze in detail the ten-dimensional $USp(32)$ model and
the six-dimensional (1,0) models.
