  We introduce the concept of inflationary twin models, that is, a class of
generalized (kinetic or "K-inflation") field theories which lead exactly to the
same cosmological evolution during the inflationary period as a given standard
scalar field theory of inflation. The twin concept permits to introduce
generalized, K-inflation theories in a controlled manner, maintaining some
inflationary predictions unaltered. Further, this concept allows to extend
analytical tools like the slow-roll expansion to the realm of K-inflation
theories, facilitating their investigation. Twins of a standard scalar field
model of inflation still may result in different results for some inflationary
observables, because of their nontrivial scalar speed of sound. This implies
that non-standard twins may lead to completely viable models of inflation, even
if their standard twin is ruled out by observations. We provide some explicit
examples of this possibility, including the physical case of a
dilaton--Dirac-Born-Infeld theory.
