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In this paper, we consider the nonlinear fractional Schrödinger equation with Sobolev critical exponent and mixed nonlinearities
(−Δ)su = λu + μ|u|q−2u + |u|2*s−2u, x ∈ RN,
∫RN |u|2dx = c, u ∈ Hs(RN), (0.1)
where s ∈ (0, 1), N > 2s, μ > 0, 2 < q < 2 + 4s/N and 2*s = 2N/N−2s is the fractional Sobolev critical exponent. We show that there exists an explicit mass threshold value c0 such that, for any c ∈ (0, c0], (0.1) admits a ground state solution u+c and a excited state solution u−c, which can be characterized as a local minimizer and as a Mountain Pass type critical point of the corresponding energy functional. We emphasis that the result is new in the case c = c0 even for the Laplacian operator, which is based on the full use of minimizers of the fractional Gagliardo-Nirenberg-Sobolev inequality and the Aubin-Talanti bubbles. Moreover, by establishing a refined upper bound of energy level, we capture the precise asymptotic behaviour of the obtained normalized solutions as c → 0+ and μ → 0+. These conclusions extend some known ones in previous papers.