  This is a continuation of the papers [4] of Bleher and Fokin and [5] of
Bleher and Liechty, in which the large $n$ asymptotics is obtained for the
partition function $Z_n$ of the six-vertex model with domain wall boundary
conditions in the disordered and ferroelectric phases, respectively. In the
present paper we obtain the large $n$ asymptotics of $Z_n$ on the critical line
between these two phases.
