  We use level truncated superstring field theory to obtain the effective
potential for the Wilson line marginal deformation parameter which corresponds
to the constant vacuum expectation value of the U(1) gauge field on the D-brane
in a particular direction. We present results for both the BPS and the non-BPS
D-brane. In the case of non-BPS D-brane the effective potential has branches
corresponding to the extrema of the tachyon potential. In the branch with
vanishing tachyon vev (M-branch), the effective potential becomes flatter as
the level of the approximation is increased. The branch which corresponds to
the stable vacuum after the tachyon has condensed (V-branch) exists only for a
finite range of values of marginal deformation parameter. We use our results to
find the mass of the gauge field in the stable tachyonic vacuum. We find this
mass to be of a non-zero value which seems to stabilize as the level
approximation is improved.
