  We compute one-loop correction to the string field theory action of the
tachyon for unstable D-branes in the framework of the boundary superstring
field theory. We would expect that the one-loop correction comes from the
partition function of the two-dimensional world-sheet theory on the annulus.
The annulus correction suggests that the genus expansion is, somehow, governed
by the effective string coupling defined in terms of the tachyon \lambda=g_s
exp(-T^2/4).
