  A BPS limit is systematically derived for straight multi- D- and DF-strings
from the D3D3bar system in the context of boundary superstring field theory.
The BPS limit is obtained in the limit of thin D(F)-strings, where the
Bogomolnyi equation supports singular static multi-D(F)-string solutions. For
the BPS multi-string configurations with arbitrary separations, BPS sum rule is
fulfilled under a Gaussian type tachyon potential and reproduces exactly the
descent relation. For the DF-strings ((p,q)-strings), the distribution of
fundamental string charge density coincides with its energy density and the
Hamiltonian density takes the BPS formula of square-root form.
