  We perform a rigorous computation of the specific heat of the Ashkin-Teller
model in the case of small interaction and we explain how the
universality-nonuniversality crossover is realized when the isotropic limit is
reached. We prove that, even in the region where universality for the specific
heat holds, anomalous critical exponents appear: for instance we predict the
existence of a previously unknown anomalous exponent, continuously varying with
the strength of the interaction, describing how the difference between the
critical temperatures rescales with the anisotropy parameter.
