  Helium-burning hot subdwarf stars of spectral types O and B (sdO/B) are
thought to be produced through various types of binary interaction. The
helium-rich hot subdwarf star EC22536-5304 was recently found to be extremely
enriched in lead. Here, we show that EC22536-5304 is a binary star with a
metal-poor subdwarf F-type (sdF) companion. We have performed a detailed
analysis of high-resolution SALT/HRS and VLT/UVES spectra, deriving metal
abundances for the hot subdwarf, as well as atmospheric parameters for both
components. Because we consider the contribution of the sdF star, the derived
lead abundance for the sdOB, +6.3 +/- 0.3 dex relative to solar, is even higher
than previously thought. We derive Teff = 6210 +/- 70 K, log g = 4.64 +/- 0.10,
[Fe/H] = -1.95 +/- 0.04, and [$\alpha$/Fe] = +0.40 +/- 0.04 for the sdF
component. Radial velocity variations, although poorly sampled at present,
indicate that the binary system has a long orbital period of about 457 days.
This suggests that the system was likely formed through stable Roche lobe
overflow (RLOF). A kinematic analysis shows that EC22536-5304 is on an
eccentric orbit around the Galactic centre. This, as well as the low
metallicity and strong alpha-enhancement of the sdF-type companion, indicate
that EC22536-5304 is part of the Galactic halo or metal-weak thick disc. As the
first long-period hot subdwarf binary below about [Fe/H] = -1, EC22536-5304 may
help to constrain the RLOF mechanism for mass transfer from low-mass,
low-metallicity red giant branch (RGB) stars to main-sequence companions.
