Consider a topologically transitive countable Markov shift
$\Sigma $
and a summable locally constant potential
$\phi $
with finite Gurevich pressure and
$\mathrm {Var}_1(\phi ) < \infty $
. We prove the existence of the limit
$\lim _{t \to \infty } \mu _t$
in the weak
$^\star $
topology, where
$\mu _t$
is the unique equilibrium state associated to the potential
$t\phi $
. In addition, we present examples where the limit at zero temperature exists for potentials satisfying more general conditions.