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NN Serpentis , an eclipsing post-common-envelope binary consisting of a white dwarf and a red dwarf , is very likely to have two planets causing variations in the period of the eclipses .
 NN Serpentis, white dwarf, red dwarf
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 NN Serpentis, white dwarf, red dwarf