Let F (X) = n≥0 (−1) εn X −λn be a real lacunary formal power series, where ε n = 0, 1 and λ n+1 /λ n > 2. It is known that the denominators Q n (X) of the convergents of its continued fraction expansion are polynomials with coefficients 0, ±1, and that the number of nonzero terms in Q n (X) is the nth term of the Stern-Brocot sequence. We show that replacing the index n by any 2-adic integer ω makes sense. We prove that Q ω (X) is a polynomial if and only if ω ∈ Z. In all the other cases Q ω (X) is an infinite formal power series, the algebraic properties of which we discuss in the special case λ n = 2 n+1 − 1.Keywords: Stern-Brocot sequence, continued fractions of formal power series, automatic sequences, algebraicity of formal power series.