(0,2) Noncritical Strings in Six Dimensions
  Type IIB strings compactified on K3 have a rich structure of solitonic strings, transforming under SO(21,5,Z). We derive the BPS tension formula for these strings, and discuss their properties, in particular, the points in the moduli space where certain strings become tensionless. By examining these tensionless string limits, we shed some further light on the conjectured dual M-Theory description of this compactification. 
Jacques Distler and Amihay Hanany