def iben_tutukov1984(history, al=1):
    """
    CE formalism from
    `Iben & Tutukov 1984, ApJ, 284, 719 <https://ui.adsabs.harvard.edu/abs/1984ApJ...284..719I/abstract>`_

    Required history parameters:
        - star_1_mass
        - star_2_mass
        - he_core_mass
        - binary_separation

    :param history: ndarray with model parameters
    :param al: alpha CE, the efficiency parameter for the CE formalism
    :return: final separation, final primary mass
    """
    M1 = history['star_1_mass'][-1]
    M2 = history['star_2_mass'][-1]
    Mc = history['he_core_mass'][-1]
    a = history['binary_separation'][-1]

    af = al * (Mc * M2) / (M1 ** 2) * a

    return af, Mc