  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.
