On marginal deformations in superstring field theory

We use level truncated superstring field theory to obtain the effective pot ential 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 th e non-BPS D-brane. In the case of non-BPS D-brane the effective potential h as branches corresponding to the extrema of the tachyon potential. In the b ranch with vanishing tachyon vev (M-branch), the effective potential become s 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 tach yonic vacuum. We find this mass to be of a non-zero value which seems to st abilize as the level approximation is improved.