Abstract : We study signatures of cosmic superstring networks containing strings of multiple tensions and Y junctions, on the cosmic microwave background (CMB) temperature and polarization spectra. Focusing on the crucial role of the string coupling constant gs, we show that the number density and energy density of the scaling network are dominated by different types of string in the gs˜1 and gs≪1 limits. This can lead to an observable shift in the position of the B-mode peak--a distinct signal leading to a direct constraint on gs. We forecast the joint bounds on gs and the fundamental string tension μF from upcoming and future CMB polarization experiments, as well as the signal to noise in detecting the difference between B-mode signals in the limiting cases of large and small gs. We show that such a detectable shift is within reach of planned experiments.