Remark on marginally stable bound states in type II string theory

The spectrum of elementary string states in type II string theory contains ultrashort multiplets that are marginally stable. $U$-duality transformation converts these states into bound states at the 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 Witten, we argue that the spectrum of the bound states of $p$-branes is in agreement with the prediction of $U$ duality.