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Wide binaries with hot subdwarf-B (sdB) primaries and main sequencecompanions are thought to form only through stable Roche lobe overflow (RLOF)of the sdB progenitor near the tip of the red giant branch (RGB). We presentthe orbital parameters of eleven new long period composite sdB binaries basedon spectroscopic observations obtained with the UVES, FEROS and CHIRONspectrographs. Using all wide sdB binaries with known orbital parameters, 23systems, the observed period distribution is found to match very well withtheoretical predictions. A second result is the strong correlation between theorbital period (P) and the mass ratio (q) in the observed wide sdB binaries. Inthe P-q plane two distinct groups emerge, with the main group (18 systems)showing a strong correlation of lower mass ratios at longer orbital periods.The second group are systems that are thought to be formed from higher massprogenitors. Based on theoretical models, a correlation between the initialmass ratio at the start of RLOF and core mass of the sdB progenitor is found,which defines a mass-ratio range at which RLOF is stable on the RGB.

The orbital period -- mass ratio relation of wide sdB+MS binaries and its application to the stability of RLOF