  In this paper, our focus is on exploring the gauge-invariant basis for
bosonic couplings within the framework of heterotic string theories,
specifically examining 3-, 5-, and 7-derivative terms. We thoroughly analyze
the invariance of these couplings under T-duality transformations and make a
notable observation: the T-duality constraint enforces the vanishing of these
couplings. We speculate that this result likely holds true for all higher
odd-derivative couplings as well. This is unlike the result in type I
superstring theory, where, for example, the couplings of 5 Yang-Mills field
strengths are non-zero. The vanishing of couplings is consistent with the
$O(d,d+16)$ symmetry of the cosmological reduction of the effective action.
