By using zeros of elliptic integrals we establish an upper bound for the number of limit cycles that emerge from the period annulus of the Hamiltonian X(H) in the system X(epsilon) = X(H) + epsilon(P, Q), where H = y2 + x4 and P, Q are polynomials in x, y, as a function of the degrees of P and Q. In particular, if (P, Q) = [GRAPHICS] with N = 2k + 1 or 2k + 2, this upper bound is k - 1.