We consider a natural generalization of the metric almost contact manifolds that we call metric f.pk-manifolds. They are Riemannian manifolds with a compatible f -structure which admits a parallelizable kernel. With some additional conditions they are called S-manifolds. We give some examples and study some properties of harmonic 1-forms on such manifolds. We also study harmonicity and holomorphicity of vector fields on them.Mathematics Subject Classification 2010: 53C15, 53C25.