We study supersymmetry breaking by Scherk-Schwarz compactifications in type I
string theory. While in the gravitational sector all mass splittings are
proportional to a (large) compactification radius, supersymmetry remains
unbroken for the massless excitations of D-branes orthogonal to the large
dimension. In this sector, supersymmetry breaking can then be mediated by
gravitational interactions alone, that are expected to be suppressed by powers
of the Planck mass. The mechanism is non perturbative from the heterotic
viewpoint and requires a compactification radius at intermediate energies of
order 10^{12}-10^{14} GeV. This can also explain the value of Newton's constant
if the string scale is close to the unification scale, of order 10^{16} GeV.Comment: 47 pages, LaTeX, typos correcte
