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Helium-burning hot subdwarf stars of spectral types O and B (sdO/B) arethought to be produced through various types of binary interaction. Thehelium-rich hot subdwarf star EC22536-5304 was recently found to be extremelyenriched in lead. Here, we show that EC22536-5304 is a binary star with ametal-poor subdwarf F-type (sdF) companion. We have performed a detailedanalysis of high-resolution SALT/HRS and VLT/UVES spectra, deriving metalabundances for the hot subdwarf, as well as atmospheric parameters for bothcomponents. Because we consider the contribution of the sdF star, the derivedlead abundance for the sdOB, +6.3 +/- 0.3 dex relative to solar, is even higherthan 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 sdFcomponent. 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 lobeoverflow (RLOF). A kinematic analysis shows that EC22536-5304 is on aneccentric orbit around the Galactic centre. This, as well as the lowmetallicity and strong alpha-enhancement of the sdF-type companion, indicatethat EC22536-5304 is part of the Galactic halo or metal-weak thick disc. As thefirst long-period hot subdwarf binary below about [Fe/H] = -1, EC22536-5304 mayhelp to constrain the RLOF mechanism for mass transfer from low-mass,low-metallicity red giant branch (RGB) stars to main-sequence companions.

EC 22536-5304: a lead-rich and metal-poor long-period binary