General signatures of superstring theories

One of the most general consequences of broad classes of superstring theories is that matter supermultiplets in the four-dimensional low-energy theory should transform as singlets and doublets with canonical electric charges under the electroweak SU(2)×U(1) group, or as singlets and fundamentals in extended groups. We discuss the generality of these and related results, and point out some indirect experimental scenarios which could exclude this class of models.