We consider the eigenfunctions of well-defined operator related to $\ensuremath{\int}{\ensuremath{\Psi}}^{*}({1}^{\ensuremath{'}},2,\dots{},N)\ifmmode\times\else\texttimes\fi{}H(1,2,\dots{},N)\ensuremath{\Psi}(1,2,\dots{},N)d(2,3,\dots{},N)$, where $\ensuremath{\Psi}$ is a configuration-interaction (CI) wave function which includes the Hartree-Fock configuration and all double excitations and $H$ is the $N$-electron Hamiltonian of an atomic or molecular system. They satisfy for the energy the same optimum convergence properties as L\"owdin's natural spin orbitals satisfy for the wave function. They are equal to the natural spin orbitals when $\ensuremath{\Psi}$ is the exact wave function or a full CI wave function; $\ensuremath{\Psi}$ may correspond to the ground state or to any excited state.