= Brane separation becomes a Wilson line
{title2=$A_i=\frac{y_i}{2\pi\alpha'}$}
Under <T-duality>, the positions $y_i$ of <D-branes> transverse to a circle of radius $R$ become diagonal <Wilson lines> on branes wrapping the dual circle $R'=\alpha'/R$. Their holonomies are $\exp(i2\pi R'A_i)=\exp(iy_i/R)$. An <Open string stretched between D-branes> with displacement $y_j-y_i+2\pi Rw$ becomes a string with shifted compact momentum $w/R'+A_j-A_i$. The stretching contribution and the shifted-momentum contribution to its squared mass are identical.
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