The orbit space
P(R8)/G, of the group
G:=SU(2)×U(1)⊂U(3) acting adjointly on the state space
P(R8) of a 3-level quantum system is discussed. The semi-algebraic structure of
P(R8)/G, is determined within the Procesi-Schwarz method. Using the integrity basis for the ring of G-invariant polynomials,
R[P(R8)]G, the set of constraints on the Casimir invariants of
U(3) group coming from the positivity requirement of Procesi-Schwarz gradient matrix,
Grad(z)⩾0, is analyzed in details.