  We study how the effect of closed-string tachyon condensation can enter into
the on-shell effective action of open-string tachyons in the bosonic case. We
also consider open-string one-loop quantum corrections to the on-shell action.
We use a sigma-model approach with boundary terms, and we utilize some results
of boundary string field theory (BSFT) to define the on-shell effective action.
We regard D-instanton-like objects with appropriate weight as closed-string
tachyon tadpoles, and we insert them into worldsheets to analyze the effect of
closed-string tachyons.
