  We present a new class of topological conformal field theories (TCFT)
characterized by a rational $W$ potential, which includes the minimal models of
A and D types as its subclasses. An explicit form of the $W$ potential is found
by solving the underlying dispersionless KP hierarchy in a particular small
phase space. We discuss also the dispersionless KP hierarchy in large phase
spaces by reformulating the hierarchy, and show that the $W$ potential takes a
universal form, which does not depend on a specific form of the solution in a
large space.
