We construct a mixed spin 1/2 and S integrable model and investigate its
finite size properties. For a certain conformal invariant mixed spin system the
central charge can be decomposed in terms of the conformal anomaly of two
single integrable models of spin 1/2 and spin (S−1/2). We also compute the
ground state energy and the sound velocity in the thermodynamic limit.Comment: This was the first correct calculation of the central charge in mixed
integrable spin chains. For effects of a magnetic field see
J.Phys.A:Math.Gen. 26 (1993) 730