EXTENDED N = 2 SUPERCONFORMAL STRUCTURE OF GRAVITY AND W-GRAVITY COUPLED TO MATTER

We show that almost all string theories, including the bosonic string, the superstring and W-string theories, possess a twisted N = 2 superc onformal symmetry. This enables us to establish a connection between t opological gravity and the field theoretical description of matter cou pled to gravity. We also show how the BRST operators of the W(n)-strin g can be obtained by hamiltonian reduction of SL(n\n-1). The tachyonic and ground ring states of W-strings are described in the light of the N = 2 superconformal structure, and the ground ring generators for th e non-critical W3-string are explicitly constructed. The relationship to G/G models and quantum integrable systems is also briefly described .