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A job listing has been spotted for a position at Microsoft which suggests that they are indeed working on a successor to the popular Xbox 360 platform.
The first job listing is for a "Graphics Hardware Architect", who will work with a team "responsible for defining and delivering next-generation console architectures from conception through implementation."
The listing indicates that the role will have responsibilities which include "architecture analysis, key technology selection, architecture specification, communication and collaboration with extended Microsoft teams and partner companies."
This is clearly the biggest indication so far that Microsoft are working on a new console, but it's unlikely they will confirm anything officially.