Evolved Stars in Binaries
The fundamental question of how stellar remnants shed their outer layers seems to have been answered for a number of cases, where the remnant has a clearly detected companion. In most cases, a stellar remnant must previously have been a red giant. At that time, when it would have been very much larger, its outer surface would have been more easily influenced by the gravitational pull of its companion. A considerable fraction of these outer layers can be stripped away and either transferred to the companion (conservative mass transfer) or partially ejected from the system (non-conservative).
HD185510An excellent example of this is the binary HD185510. This star consists of a hot subdwarf and a cool (K) giant. The stars orbit one another with a period of 21 days. Interestingly, the cool star also has spots and rotates with a slightly longer period (22 days). While it is visible in optical and infrared light, the hotter subdwarf can only be seen in ultraviolet light. The figure shows a simulation of how the system might appear if you could see in both optical and ultraviolet light at the same time. Click on the figure to see a movie simulating how the stars orbiting one another. More details can be found in the paper Timing the eclipse in HD185510 by C.S.Jeffery and T. Simon. The physical dimensions of the two stars are compared in the following table.
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