The paper is devoted to the problem of finding the exact constant W2∗ in the inequality ‖f‖ ≤ K ⋅ ω2(f, 1) for bounded functions f with the property∫kk+1f(x)dx=0,k∈ℤ. Our approach allows us to reduce the known range for the desired constant as well as the set of functions involved in the extremal problem for finding the constant in question. It is shown that W2∗ also turns out to be the exact constant in a related Jackson–Stechkin type inequality.