  Long-orbital-period subdwarf B (sdB) stars with main-sequence companions are
believed to be the product of stable Roche Lobe overflow (RLOF), a scenario
challenged by recent observations. Here we represent the results of a
systematic study of the orbital-period distribution of sdB binaries in this
channel using detailed binary evolution calculations. We show that the observed
orbital-period distribution of long-period sdB binaries can be well explained
by this scenario. Furthermore, we find that, if the progenitors of the sdB
stars have initial masses below the helium flash mass, the sdB binaries
produced from stable RLOF follow a unique mass -- orbital period relation for a
given metallicity $Z$; increasing the orbital period from $\sim 400$ to $\sim
1100$\,d corresponds to increasing the mass of the sdB star from $\sim 0.40$ to
$\sim 0.49\,M_\odot$ for $Z=0.02$. We suggest that the longest sdB binaries
(with orbital period $> 1100$\,d) could be the result of atmospheric RLOF. The
mass -- orbital period relation can be tested observationally if the mass of
the sdB star can be determined precisely, e.g.\ from asteroseismology. Using
this relation, we revise the orbital period distribution of sdB binaries
produced by the first stable RLOF channel for the best fitting model of Han et
al (2003), and show that the orbital period has a peak around 830\,d.
