  We show that one can construct a classical affine W-algebra via a classical
BRST complex. This definition clarifies that classical affine W-algebras can be
considered as quasi-classical limits of quantum affine W-algebras.
  We also give a definition of a classical affine fractional W-algebra as a
Poisson vertex algebra. As in the classical affine case, a classical affine
fractional W-algebra has two compatible $\lambda$-brackets and is isomorphic to
an algebra of differential polynomials as a differential algebra. When a
classical affine fractional W-algebra is associated to a minimal nilpotent, we
describe explicit forms of free generators and compute $\lambda$-brackets
between them. Provided some assumptions on a classical affine fractional
W-algebra, we find an infinite sequence of integrable systems related to the
algebra, using the generalized Drinfel'd and Sokolov reduction.
