def demarco2011(history, al=1, lb=1):
    """
    CE formalism from
    `De Marco et al. 2011, MNRAS, 411, 2277 <https://ui.adsabs.harvard.edu/abs/2011MNRAS.411.2277D/abstract>`_

    Required history parameters:
        - star_1_mass
        - star_2_mass
        - he_core_mass
        - binary_separation
        - rl_1

    :param history: ndarray with model parameters
    :param al: alpha CE, the efficiency parameter for the CE formalism
    :param lb: lambda CE, the mass distribution factor of the primary envelope: lambda * Rl = the effective
               mass-weighted mean radius of the envelope at the start of CE.
    :return: final separation, final primary mass
    """
    M1 = history['star_1_mass'][-1] # Msun
    M2 = history['star_2_mass'][-1] # Msun
    Mc = history['he_core_mass'][-1] # Msun
    Me = M1 - Mc # Msun
    a = history['binary_separation'][-1] # Rsun
    Rl = history['rl_1'][-1]  # Rsun

    af = (a * al * lb * Rl * Mc * M2) / (Me * (Me / 2.0 + Mc) * a + al * lb * Rl * M1 * M2)

    return af, Mc