and $\sim2.5$ [@Nelemans05]. At the same time, they showed that no single value of $\alpha_\mathrm{CE}\lambda$ can satisfy all the merger scenarios they considered. The values that can reconstruct the observed systems scatter from unphysically negative to suspiciously large $\sim 2$, with values for $\alpha_\mathrm{CE}\lambda$ skewed to low ($\la 0.5$) values. We note that those low values are rather in agreement with expected $\alpha_\mathrm{CE}<1$ and $\lambda$ variation along the giant branch. The considered systems had primary masses from $1.2$ to $3\,M_\odot$, and most of the likely progenitors clustered around $\sim 1.7\,M_\odot$.

Orbital evolution during the mass transfer {#sec:2.2}
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Assuming a circular orbit, the instantaneous angular momentum about the binary’s centre of mass is defined as $$\label{eq1}
J = \frac{m_\mathrm{d} m_\mathrm{a}}{\sqrt{m_\mathrm{d} + m_\mathrm{a}}} \sqrt{Ga},
\label{oam}$$ where $a$ is the current orbital separation.

Neglecting the spin angular momentum of the companions, a general equation for the orbital evolution of a non-eccentric binary is: $$\label{oam_der}
2 \frac{\dot J}{J} = \frac{\dot a}{a} + 2 \frac{\dot m_\mathrm{d}}{m_\mathrm{d}}+ 2 \frac{\dot m_\mathrm{a}}{m_\mathrm{a}}
-  \frac{\dot m_\mathrm{d}+ \dot m_\mathrm{a}}{ m_\mathrm{d}+m_\mathrm{a}}.$$

Here $\dot J$ and $\dot a$ are the rates of change of the orbital angular momentum and of the orbital separation, respectively. The quantities $\dot m_\mathrm{d}$ and $\dot m_\mathrm{a}$ 