Over the past year, rumors have made it well known that Microsoft is looking to make Windows 10 more modular with a base Windows Core OS. Andromeda OS, perhaps for a dual screen folding Surface Phone and Polaris OS, a new stripped down UWP version of Windows 10 for new devices, also happens to be the two rumored new operating systems which are part of the modular core.
In the latest developments, (via MSPoweruser,) Microsoft leaker Walking Cat revealed that the latest Windows Insider builds contain entries for these alternative Windows 10 Shells, perhaps getting ready for the long-awaited Surface Phone.
According to WalkingCat, the list comes from the .obj files inside composers inside build 17025. You can see the references to the shells for both Andromeda and Polaris. There is also references to shells for Surface Hub and IoT. WalkingCat, though, also believes that Andromeda is not a "phone" but rather a new category named 'HingedDualScreenDevice.'
Obviously, there is not much to the list, but it does perhaps suggest that Polaris and Andromeda are in fact in development at Microsoft. Here's to hoping devices running these operating systems come to light soon.