What are non maximally entangled Bell bases?

One such nonmaximally entangled basis might be:

$|Theta^1 rangle = |Psi^+ rangle = dfrac{|01 rangle + |10rangle}{sqrt{2}}$

$|Theta^2 rangle = |Psi^- rangle = dfrac{|01 rangle - |10rangle}{sqrt{2}}$

$|Theta^3 rangle = dfrac{ cos(phi)^2|00 rangle + sin(phi)^2|11rangle}{sqrt{cos(phi)^4 + sin(phi)^4 } } $

$|Theta^4 rangle = dfrac{ cos(phi)^2|00 rangle - sin(phi)^2|11rangle}{sqrt{cos(phi)^4 + sin(phi)^4 } } $

according to the paper "Nonmaximally entangled bases and their application in entanglement purification via swapping".