  Recently, Kim, Shiu and Vafa proposed general consistency conditions for six
dimensional supergravity theories with minimal supersymmetry coming from
couplings to strings. We test them in explicit perturbative orientifold models
in order to unravel the microscopic origin of these constraints. Based on the
perturbative data, we conjecture the existence of null charges $Q \cdot Q=0 $
for any six-dimensional theory with at least one tensor multiplet, coupling to
string defects of charge $Q$. We then include the new constraint to exclude
some six-dimensional supersymmetric anomaly-free examples that have currently
no string or F-theory realization. We also investigate the constraints from the
couplings to string defects in case where supersymmetry is broken in tachyon
free vacua, containing non-BPS configurations of brane supersymmetry breaking
type, where the breaking is localized on antibranes. In this case, some
conditions have naturally to be changed or relaxed whenever the string defects
experience supersymmetry breaking, whereas the constraints are still valid if
they are geometrically separated from the supersymmetry breaking source.
