  We consider d=4 N=2 supergravity theories which serve as low-energy effective
actions for heterotic strings on K_3 \times T^2. At the perturbative level we
construct a new version of the heterotic effective action in which the axion
has been traded for an antisymmetric tensor field. In the string frame the
antisymmetric tensor doesn't transform under Poincar\'e supersymmetry into the
dilaton-dilatini system. This indicates that in this frame the antisymmetric
tensor field and the dilaton are not contained in an N=2 vector-tensor
multiplet. Instead, we find that the heterotic dilaton is part of a
compensating hypermultiplet, whereas the antisymmetric tensor is part of the
gravitational multiplet. In order to obtain our results we use superconformal
techniques. This enables us to comment on the range of applicability of this
particular framework.
