  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.
