  We study the phenomenology of cosmic-rays (CRs) at the galactic/extragalactic
transition, focusing on two opposite models for the composition of the
extragalactic (EG) component. Model A assumes a mixed source composition, with
nuclear abundances similar to that of the low-energy CRs, while model B assumes
that the EG sources accelerate only protons. We study the limits within which
both scenarios can reproduce the high-energy CR data, from the point of view of
the energy spectrum, each with its own interpretation of the ankle (as the
GCR/EGCR transition for model A; as a pair-production dip for model B) and its
own source spectrum (in E?2.3 model A; in E?2.6 or E?2.7 for model B). We then
focus on the energy evolution of the high-energy CR composition and compare our
predictions for two main composition-related observables (Xmax and hlnAi) with
the available data, to conclude that model A is currently favoured.
Uncertainties are discussed and distinctive features of the two models are
identified, which should allow one to distinguish between the models in the
near future when more precise measurements are available with higher-statistics
experiments.
