  While R-R tadpoles should be canceled for consistency, string models with
broken supersymmetry generally have uncanceled NS-NS tadpoles. Their presence
signals that the background does not solve the field equations, so that these
models are in "wrong" vacua. In this letter we investigate, with reference to
some prototype examples, whether the true values of physical quantities can be
recovered resumming the NS-NS tadpoles, hence by an approach that is related to
the analysis based on String Field Theory by open-closed duality. We show that,
indeed, the positive classical vacuum energy of a Dp-brane of the bosonic
string is exactly canceled by the negative contribution arising from tree-level
tadpole resummation, in complete agreement with Sen's conjecture on open-string
tachyon condensation and with the consequent analysis based on String Field
Theory. We also show that the vanishing classical vacuum energy of the SO(8192)
unoriented bosonic open-string theory does not receive any tree-level
corrections from the tadpole resummation. This result is consistent with the
fact that this (unstable) configuration is free from tadpoles of massless
closed-string modes, although there is a tadpole of the closed string tachyon.
The application of this method to superstring models with broken supersymmetry
is also discussed.
