  The transition from the ordered commensurate phase to the incommensurate
gaussian phase of the antiferroelectric asymmetric six-vertex model is
investigated by keeping the temperature constant below the roughening point and
varying the external fields $(h,v)$. In the $(h,v)$ plane, the phase boundary
is approached along straight lines $\delta v=k \delta h$, where $(\delta
h,\delta v)$ measures the displacement from the phase boundary. It is found
that the free energy singularity displays the exponent 3/2 typical of the
Pokrovski-Talapov transition $\delta f \sim const (\delta h)^{3/2}$ for any
direction other than the tangential one. In the latter case $\delta f$ shows a
discontinuity in the third derivative.
