Abstract With an effective temperature of ≃200,000 K, KPD 0005+5106 is one of the hottest white dwarfs (WDs). ROSAT unexpectedly detected “hard” (∼1 keV) X-rays from this apparently single WD. We have obtained Chandra observations that confirm the spatial coincidence of this hard X-ray source with KPD 0005+5106. We have also obtained XMM-Newton observations of KPD 0005+5106, as well as PG 1159−035 and WD 0121−756, which are also apparently single and whose hard X-rays were detected by ROSAT at 3σ–4σ levels. The XMM-Newton spectra of the three WDs show remarkably similar shapes that can be fitted by models including a blackbody component for the stellar photospheric emission, a thermal plasma emission component, and a power-law component. Their X-ray luminosities in the 0.6–3.0 keV band range from 4 × 10²⁹ to 4 × 10³⁰ erg s⁻¹. The XMM-Newton EPIC-pn soft-band (0.3–0.5 keV) light curve of KPD 0005+5106 is essentially constant, but the hard-band (0.6–3.0 keV) light curve shows periodic variations. An analysis of the generalized Lomb–Scargle periodograms for the XMM-Newton and Chandra hard-band light curves finds a convincing modulation (false-alarm probability of 0.41%) with a period of 4.7 ± 0.3 hr. Assuming that this period corresponds to a binary orbital period, the Roche radii of three viable types of companion have been calculated: M9V star, T brown dwarf, and Jupiter-like planet. Only the planet has a size larger than its Roche radius, although the M9V star and T brown dwarf may be heated by the WD and inflate past the Roche radius. Thus, all three types of companion may be donors to fuel accretion-powered hard X-ray emission.