  We show that there are no vacuum solutions with a purely magnetic Weyl tensor
with respect to an observer submitted to kinematic restrictions involving first
order differential scalars. This result generalizes previous ones for the
vorticity-free and shear-free cases. We use a covariant approach which makes
evident that only the Bianchi identities are used and, consequently, the
results are also valid for non vacuum solutions with vanishing Cotton tensor.
