The beautiful cutaway drawing included in this answer shows a metallic turnip-shaped object attached to the bottom of the Ariane V core stage.
What is that object?
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It's an Helium tank; nicknamed LHESS
This website capcomespace.net have a very good article on the subject (in french)
Extract (roughly translated):
The tanks pressurisation system preserve the tanks structural integrity, and a continuous propellant flow. The LOX tank is pressurised by 141kg of liquid helium stored at 7°K and 19 bar, warmed up by contact with the turbine exhaust. The flow is 0,2 kg-s, regulated by vanes on a rotatory disc. The H2 tank pressurisation is done with gaseous H2.
Also seen on this schematic with a similar label: color legend: In light blue, the pressurisation line running up to the LOX tank, dark blue is the Liquid Helium tank. Green are He tank too, but used for the gimbal system
AirLiquide, the tank maker has a product page describing it here
Proven experience ǀ LHESS supercritical helium tanks have been fitted on Ariane 5 since its first launch in 1996, thereby proving their efficiency and robustness. Moreover, they have been continuously improved to increase their performance