It is a joke. According to the mythology, there were seven Pleiads, one of which became mortal and wasn't placed into the sky. Now they're find a star that may have been a part of the cluster, so it is the lost "seventh sister".

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The diameter of a white dwarf or a neutron star depends on how massive it is. More massive the object is, the smaller is its diameter. Sirius B, which is a relatively massive white dwarf, has a diameter about that of the Earth. According to Ken Croswell's webpage, the diameter of GD 50 is only 54% of Earth's diameter!

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman