Spectrum of elementary string states in type II string theory contains
ultra-short multiplets that are marginally stable. U-duality transformation
converts these states into bound states at threshold of p-branes carrying
Ramond-Ramond charges, and wrapped around p-cycles of a torus. We propose a
test for the existence of these marginally stable bound states. Using the
recent results of Polchinski and of Witten, we argue that the spectrum of bound
states of p-branes is in agreement with the prediction of U-duality.Comment: LaTeX file, 6 page