  We discuss the large order behaviour and Borel summability of the topological
expansion of models of 2D gravity coupled to general $(p,q)$ conformal matter.
In a previous work it was proven that at large order $k$ the string
susceptibility had a generic $a^k\Gamma(2k-\ud)$ behaviour. Moreover the
constant $a$, relevant for the problem of Borel summability, was determined for
all one-matrix models. We here obtain a set of equations for this constant in
the general $(p,q)$ model. String equations can be derived from the
construction of two differential operators $P,Q$ satisfying canonical
commutation relations $[P,Q]=1$. We show that the equation for $a$ is
determined by the form of the operators $P,Q$ in the spherical or semiclassical
limits. The results for the general one-matrix models are then easily
recovered. Moreover, since for the $(p,q)$ string models such $p=(2m+1)q\pm1$
the semiclassical forms of $P,Q$ are explicitly known, the large order
behaviour is completely determined. This class contains all unitary $(q+1,q)$
models for which the answer is specially simple. As expected we find that the
topological expansion for unitary models is not Borel summable.
\preprint{SPhT/92-163}, Plain-TeX, macro harvmac
