Barotropic index w-singularities in cosmology

We find an explicit cosmological model which allows a special type of cosmological singularity which we call a $w$-singularity. This singularity has the scale factor finite, the energy density and pressure vanishing, and the only singular behaviour appears in a time-dependent barotropic index $w(t)$. It is different from the type IV cosmological singularity in that it does not exhibit the divergence of the higher derivatives of the Hubble parameter and from the big-brake since it does not fulfill the anti-Chaplygin gas equation of state. We also find an interesting duality between the $w$-singularities and the big-bang singularities. Physical examples of $w$-singularities appear in $f(R)$, scalar field and brane cosmologies.