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Context. A large number of low-mass ( < 0.20 M ) helium white dwarfs (He WDs) have recently been discovered. The majority of these are orbiting another WD or a millisecond pulsar (MSP) in a close binary system; a few examples are found to show pulsations or to have a main-sequence star companion. There appear to be discrepancies between the current theoretical modelling of such lowmass He WDs and a number of key observed cases, indicating that their formation scenario yet remains to be fully understood.
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