  We study 2x2 matrices A such that the corresponding TBA equations yield c[A]
in the form of the effective central charge of a minimal Virasoro model.
Certain properties of such matrices and the corresponding solutions of the TBA
equations are established. Several continuous families and a discrete set of
admissible matrices A are found. The corresponding two-term dilogarithm
identities (some of which appear to be new) are obtained. Most of them are
proven or shown to be equivalent to previously known identities.
