def mass2_from_tau0_tau3(tau0, tau3, f_lower):
    r"""Returns the secondary mass from the given :math:`\tau_0, \tau_3`."""
    mtotal = mtotal_from_tau0_tau3(tau0, tau3, f_lower)
    eta = eta_from_tau0_tau3(tau0, tau3, f_lower)
    return mass2_from_mtotal_eta(mtotal, eta)
r"""Returns the secondary mass from the given :math:`\tau_0, \tau_3`.