Noncommutative open string theories and their dualities

The recently found noncritical open string theories is reviewed. These open strings, noncommutative open string theories (NCOS), arise as consistent quantum theories describing the low energy theory of D-branes in a background electric B-field in the critical limit. Focusing on the D3-brane case, we construct the most general (3 + 1) NCOS, which is described by four parameters. We study S- and T-dualities of these theories and argue the existence of a U-duality group.