Finite sequences representing expected order statistics

Characterizations of finite sequences ${\beta }_1<\cdots <{\beta }_n$ representing expected values of order statistics from a random sample of size $n$ are given. As a by-product, a characterization of binomial mixtures, when the mixing random variable is supported in the open interval $(0,1)$, is presented; this enables the exact description of the convex hull of the open binomial curve, as well as the open moment curve.