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==Noninteracting white dwarfs==
The youngest, hottest WD, [[KPD 0005+5106]], is very close to 100,000 K, of type DO and is the first single WD recorded as an X-ray source with ROSAT.<ref name=Fleming>{{ cite journal |author=Fleming TA|author2=Werner K |author3=Barstow MA |journal= The Astrophysical Journal|date=October 1993 |volume=416 |pages=L79 |title=Detection of the First Coronal X-Ray Source about a White Dwarf |bibcode=1993ApJ...416L..79F |doi=10.1086/187075 |doi-access=free }}</ref><ref name=Werner1994>{{ cite journal |title=Spectral analysis of the hottest known helium-rich white dwarf: KPD 0005+5106 |author=Werner |journal= Astronomy and Astrophysics|date=1994 |volume=284 |pages=907 |bibcode = 1994A&A...284..907W }}</ref>
==Cataclysmic variables==
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X-rays from magnetic cataclysmic variables are common because accretion provides a continuous supply of coronal gas.<ref name=Trimble/> A plot of number of systems vs. orbit period shows a statistically significant minimum for periods between 2 and 3 hr which can probably be understood in terms of the effects of magnetic braking when the companion star becomes completely convective and the usual dynamo (which operates at the base of the convective envelope) can no longer give the companion a magnetic wind to carry off angular momentum.<ref name=Trimble/> The rotation has been blamed on asymmetric ejection of planetary nebulae and winds<ref name=Spruit>{{ cite journal |title=Origin of the rotation rates of single white dwarfs |author=Spruit HC |date=1998 |journal= Astronomy and Astrophysics|volume=333 |pages=603 |arxiv = astro-ph/9802141 |bibcode = 1998A&A...333..603S }}</ref> and the fields on in situ dynamos.<ref name=Schmidt97>{{ cite journal |doi=10.1086/304746 |author=Schmidt GD |author2=Grauer AD |date=1997 |title=Upper Limits for Magnetic Fields on Pulsating White Dwarfs |journal= The Astrophysical Journal|volume=488 |issue=2 |pages=827–830 |bibcode=1997ApJ...488..827S|doi-access=free }}</ref> Orbit and rotation periods are synchronized in strongly magnetized WDs.<ref name=Trimble/> Those with no detectable field never are synchronized.
With temperatures in the range 11,000 to 15,000 K, all the WDs with the most extreme fields are far too cool to be detectable EUV/X-ray sources, e.g., Grw +70°8247, LB 11146, SBS 1349+5434, PG 1031+234 and GD 229.<ref name=Schmidt95>{{ cite journal |doi=10.1086/175962 |author=Schmidt GD |author2=Smith PS |title=A Search for Magnetic Fields among DA White Dwarfs |journal= The Astrophysical Journal|date=1995 |volume=448 |pages=305 |bibcode=1995ApJ...448..305S|doi-access=free }}</ref>
Most highly magnetic WDs appear to be isolated objects, although G 23–46 (7.4 MG) and LB 1116 (670 MG) are in unresolved binary systems.<ref name=Barstow/>
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