  We show that a complete gradient Ricci soliton $(M^n,\,g)$ with constant
scalar curvature and a non-parallel closed conformal vector field is isometric
to either the Euclidean space, or an Euclidean sphere, or negatively Einstein
warped product of the real line with a complete non-positively Einstein
manifold. Moreover, we show that a K\"ahler gradient Ricci soliton of real
dimension $\ge 4$, with a non-parallel closed conformal real vector field is
Ricci-flat (Calabi-Yau) and is flat in dimension 4. Finally, we show that a
Ricci soliton whose associated 1-form is harmonic, has constant scalar
curvature.
