Spectrum and separability of mixed 2-qubit X-states
  The separable mixed 2-qubit X-states are classified in accordance with degeneracies in the spectrum of density matrices. It is shown that there are four classes of separable X-states, among them: one 4D family, a pair of 2D family and a single, zero-dimensional maximally mixed state. 
Arsen Khvedelidze and Astghik Torosyan
quant-ph