de Havilland Sea Vixen Cold War Jet Aircraft

Have a look at the offset cockpit layout they had on the de Havilland Sea Vixen. With the wings folded like this, you can also see just how big the external fuel tanks were on this aircraft. The de Havilland Sea Vixen, a twin-engine jet fighter specifically designed for fleet defence and strike operations, graced the decks of Royal Navy aircraft carriers like HMS Ark Royal and HMS Eagle. Its maiden flight was in 1951 and it would eventually be replaced by more advanced aircraft in the early 1970s, including the American-made McDonnell Douglas F4 Phantom and the British-manufactured Blackburn Buccaneer.

Can anybody tell me what the round object is here, between the engine’s exhausts?