We report the discovery of short-period photometric variability and modulated Zeeman-split hydrogen emission in SDSSJ125230.93-023417.72 ( EPIC 228939929 ) , a variable white dwarf star observed at long cadence in K2 Campaign 10 .
The behavior is associated with a magnetic ( B = 5.0 MG ) spot on the stellar surface , making the 317.278 -second period a direct measurement of the stellar rotation rate .
This object is therefore the fastest-rotating apparently isolated ( without a stellar companion ) white dwarf yet discovered , and the second found to exhibit chromospheric Balmer emission after GD 356 , in which the emission has been attributed to a unipolar inductor mechanism driven by a possible rocky planet .
We explore the properties and behavior of this object , and consider whether its evolution may hold implications for white dwarf mergers and their remnants .