  Our results in this paper are two-fold. First, we consider current
fluctuations of the stationary asymmetric simple exclusion process (ASEP), run
for some long time $T$, and show that they are of order $T^{1 / 3}$ along a
characteristic line. Upon scaling by $T^{1 / 3}$, we establish that these
fluctuations converge to the long-time height fluctuations of the stationary
KPZ equation, that is, to the Baik-Rains distribution. This result has long
been predicted under the context of KPZ universality and in particular extends
upon a number of results in the field, including the work of Ferrari and Spohn
in 2005 (who established the same result for the TASEP), and the work of Balazs
and Seppalainen in 2010 (who established the $T^{1 / 3}$ scaling for the
general ASEP).
  Second, we introduce a class of translation-invariant Gibbs measures that
characterizes a one-parameter family of slopes for an arbitrary ferroelectric,
symmetric six-vertex model. This family of slopes corresponds to what is known
as the conical singularity (or tricritical point) in the free energy profile
for the ferroelectric six-vertex model. We consider fluctuations of the height
function of this model on a large grid of size $T$ and show that they too are
of order $T^{1 / 3}$ along a certain characteristic line; this confirms a
prediction of Bukman and Shore from 1995 stating that the ferroelectric
six-vertex model should exhibit KPZ growth at the conical singularity.
  Upon scaling the height fluctuations by $T^{1 / 3}$, we again recover the
Baik-Rains distribution in the large $T$ limit. Recasting this statement in
terms of the (asymmetric) stochastic six-vertex model confirms a prediction of
Gwa and Spohn from 1992.
