  In this work, we investigate an exactly solvable two-leg spin ladder with
three-spin interactions. We obtain analytically the finite-size corrections of
the low-lying energies and determine the central charge as well as the scaling
dimensions. The model considered in this work has the same universality class
of critical behavior of the XX chain with central charge c=1. By using the
correlation matrix method, we also study the finite-size corrections of the
Renyi entropy of the ground state and of the excited states. Our results are in
agreement with the predictions of the conformal field theory.
