  We investigate the stability of a new class of BPS cosmic strings in N=1
supergravity with D-terms recently proposed by Blanco-Pillado, Dvali and Redi.
These have been conjectured to be the low energy manifestation of D-strings
that might form from tachyon condensation after D- anti-D-brane annihilation in
type IIB superstring theory. There are three one-parameter families of
cylindrically symmetric one-vortex solutions to the BPS equations (tachyonic,
axionic and hybrid). We find evidence that the zero mode in the axionic case,
or s-strings, can be excited. Its evolution leads to the decompactification of
four-dimensional spacetime at late times, with a rate that decreases with
decreasing brane tension.
