Restrictions to neutron star properties based on twin-peak   quasi-periodic oscillations
  We consider twin-peak quasi-periodic oscillations observed in the accreting low-mass neutron star binaries and explore restrictions to central compact object properties that are implied by various QPO models. For each model and each source, the consideration results in a specific relation between the compact object mass $M$ and the angular-momentum $j$ rather than in their single preferred combination. Moreover, restrictions on the models resulting from observations of the low-frequency sources are weaker than those in the case of the high-frequency sources. 
Gabriel Torok, Pavel Bakala, Eva Sramkova, Zdenek Stuchlik, Martin   Urbanec and Katerina Goluchova
astro-ph.HE