How to describe the spin state of a massless particle?

For a massive particle, say, an electron, we can use a spin vector ##\vec{S}## to denote its spin state, and the corresponding covariant four vector ##S^\mu##. And we have ##P \cdot S = 0## in the particle rest frame, so it holds in any frame for it is an Lorentz scalor.

But, what if the particle is massless, so that it has no rest frame, how do we describe its spin state? I think ##\vec{S}## has no definition in this case.



Regards!