In this image of the Ariane 4, you can see a blue shroud around the second stage.
At least I think it's a shroud. It is present in many photos of Ariane 4 launches, but the drawings I've seen omit it. What is the purpose of this object?
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Not a complete answer (update in the footer!) because I wasn't able to find any official explanation, but I did find enough that I thought you might be interested in my findings so far. It also gets more and more intriguing, check this out:
Ariane 44L launching SICRAL 1 and Skynet 4F military satellites on its V139 mission (Source: CapComEspace)
So this second stage shroud must be some sort of thermal insulation that is only added to the launcher on fueling up the stage tanks (it uses nitrogen tetroxide oxidizer and UH25 that is a mixture of UDMH with 25% hydrazine as fuel) and this shroud is collapsed from the launcher a few seconds into flight. Here is a video of Ariane V72 launch sequence (relevant part starts at 30 minutes into the video) from ESA pages that shows this process in motion.
It's also worth mentioning that this shroud was used on all Ariane rocket family from 1 to 4, since they all used same second stage bipropellant that was precooled before the tanks are fueled:
Second stages of Ariane are nitrogen tetroxide and unsymmetrical dimethyl hydrazine (UDMH), which, unlike cryogenic fuel, do not require thermal insulation. But because the propellants occupy a larger tank volume when they are warm, the nitrogen tetroxide and UDMH are precooled before being loaded. First and second-stage fuelling takes place within the tower, where the air-conditioning helps to keep the propellants cool.
Source: Archived copy of FLIGHT International magazine from 17 February 1979 (PDF)
So this must be something similar to what is used on Minotaur 1 and referred to as the banana because of the color and since it is peeled away in sections during launch and while the rocket is clearing the launch tower:
Minotaur I rocket during launch on its NROL-66 mission showing the first stage banana (Source: SpaceflightNow)
Here are a few other things I was able to learn about the mysterious Ariane 4 second stage insulation:
The material these tiles are made of remains a mystery to me, but they do appear light and the front face is highly reflective so I'd imagine some low-density EPDM-based thermal insulation covered with layers of aluminum foil and lightly framed might be used. This all seems to be related to second stage propellant stability and performance, keeping it thermally stable, preventing water vapor condensation and similar potential problems before the launch in the hot and humid climate of the Kourou, Guiana Space Centre.
Here's one more photo from a daytime launch of Ariane 4 (configuration 40, no stage 0 SRBs) on Flight 75 (Source: Arianespace Ariane heritage multimedia library):
Anyway, I wanted to post my findings here to hopefully help someone find official explanation behind this shroud (my French-fu is rusty and weak), what it's made of, when exactly is it attached to the second stage, how it is released (the use of the pull-cable is my educated guess based on photographs) and if it was indeed used on all launches of the Ariane family of rockets from 1 to 4.
Update: Thanks to @Hobbes and his superior French-fu skills we now have a confirmation (machine translation, a bit rough around the edges):
Before takeoff, while waiting on the launch platform, the tanks of the second stage are protected by a thermal cover made of polystyrene, ventilated with cold air, which limits the heat exchange between the propellant and the external environment. The evaporated N2O4 is above 20 °C. This thermal cover is dropped off during the launch, pulled by cables attached to the umbilical tower.
So there we have it, it is indeed a thermal shroud, or a cover made of light insulator (polystyrene) that must be covered by a reflective foil or paint on the outer side (all the photographs show it in colors of the foreground it reflects), and serves as a protection against environmental heat and keeping the precooled second stage propellants nicely chilled to prevent in-tank thermal expansion of the oxidizer (N2O4 starts evaporating above 20°C). The tiles are indeed collapsed off the launcher by a cable that is attached to the umbilical tower on rocket launch. As a side note and perhaps another interesting information that isn't mentioned there, that the umbilical lines are released by explosive charges.