Relation between the orbital period and the metallicity for the sdB binaries (left) and He-WD binaries (right) in our sample. The relation is a result of the donor radius on the RGB being strongly sensitive to its metallicity. The period-metallicity relation also depends on the sdB/He-WD mass, which is shown by the colour scale. The systems that are recognizable as composite sdBs are shown in filled circles while the single-lined ones are shown in grey open circles. For the He-WDs no distinction is made between composite and single-lined systems.