Conformal transformation in f(T) theories

It is well known that f(R) theories are dynamically equivalent to a particular class of scalar-tensor theories. In analogy to the f(R) extension of the Einstein-Hilbert action of general relativity, f(T) theories are generalizations of the action of teleparallel gravity. The field equations are always second order, remarkably simpler than f(R) theories. It is interesting to investigate whether f(T) theories have the similar conformal features possessed in f(R) theories. It is shown, however, that f(T) theories are not dynamically equivalent to teleparallel action plus a scalar field via conformal transformation, since there appears an additional scalar-torsion coupling term. We discuss briefly what constraint of this coupling term may be put on f(T) theories from observations of the solar system