Hot helium-rich white dwarfs (spectral type DO) must be much hotter than DAs to emit soft X-rays blueward of the He II ground state edge (<228 A). Only at Teff as high as 100 000 K the He II opacity is sufficiently reduced because the ionisation balance shifts in favor of He III. Therefore, only two of the hottest (previously known) DOs were detected in the ROSAT PSPC survey : PG1034+001 (which was also detected by EXOSAT , Teff = 100 000 K) and KPD 0005+5106, the hottest known DO white dwarf (Teff = 120 000 K). The PSPC spectrum of the latter was interpreted as emission from a relatively cool corona about the WD . A recent analysis of Chandra observations employing new model atmospheres, however, suggests a thermal photospheric origin of the observed 20-80 A flux .
One out of two cooler DOs that lies in a region with very low interstellar HI density was discovered in the ROSAT WFC survey through the detection of EUV flux redward of the He II 228 A edge: RE J0503-289  (Teff = 70 000 K). This object is remarkable because it is the only DO detected in both the WFC and EUVE sky surveys. Although many strong absorption features can be identified in the EUVE spectrum and attributed to He, C, and O, the spectral fit with models is unsatisfactory . The main problem is the strong blanketing by nickel lines of which the majority has inaccurately known wavelength positions. Also, an unknown mix of other metals probably affects the EUV flux distribution. Fits to the EUV spectrum give results for Teff and metal abundances that are inconsistent with optical and UV spectra. Ultimately, self-consistent diffusion models like in the case of DAs should be attempted; however, these models up to now completely fail to explain the metal abundances observed in the UV spectra of DO white dwarfs .
Only one other DO was detected in the EUV at X > 228 A, namely HD149499B, although it escaped detection in the WFC and EUVE sky surveys. This WD is important because it is the coolest known DO, and in the HRD it is located at the hot boundary of the DB gap in which no He-rich WD is known. Hence it marks the location where He-rich atmospheres are transformed into H-rich ones and in fact, this WD contains a significant amount of hydrogen in its atmosphere (H/He = 0.22). The reason for the occurrence of the DB gap is a longstanding and still unanswered question. Therefore, the precise parameters of this star are of considerable interest, but optical spectroscopy is problematic because of the bright companion. Flux was detected in an EUVE spectrum at 230-360 A and Teff could be determined with very high precision (49 500 K ). This result is in full agreement with optical and FUV spectroscopy so that the situation is satisfactory, in contrast to that of RE J0503-289 discussed earlier. Apparently, the surprisingly low metal content in HD149499B allows to derive consistent results with current models composed only of H and He.
PG1159 stars, the DO progenitors, have high C and O abundances in addition to helium in their photospheres so that they are thermal X-ray emitters only at even higher temperatures (Teff > 140 000 K). Eight objects were detected in the ROSAT PSPC survey, two are remarkable new discoveries, which turned out to be among the hottest objects of their class . One of them, RX J2117.1+3412, is a bright central star of a planetary nebula and still very luminous (about 104 L0) and hot (Teff = 170 000 K). It is a nonradial g-mode pulsator (a GW Vir variable) and became a prime target for asteroseismology analyses . PG 1520+525, also a central star, is the only PG 1159 star (except for H1504+65, see below) for which a reasonable
EUVE spectrum could be obtained. Flux is detected in a very narrow region, 100130 A, and it was used to estimate reff = 150 000 K. The star is a nonpulsator so that this result in combination with the temperature of the pulsating prototype PG 1159035 (= GW Vir, 140 000 K) constrains the location of the blue edge of the GW Vir instability strip in the HRD . This result was confirmed later by a Chandra LETG observation .
The most bizarre X-ray emitting WD is H1504+65. It was discovered as one of the brightest soft X-ray sources in the sky in the HEAO-1 survey . Optical follow-up spectroscopy revealed a hot WD closely related to the PG 1159 class. It is the hottest WD known (reff = 200 000 K) and has a unique surface photospheric composition. It is H- and He-free and mainly composed of C and O by equal parts. The EUVE spectrum shows flux in the 75-150 A range and is dominated by strong and broad O VI and Ne VII absorption lines . Chandra has taken a spectacular LETG spectrum, arguably the richest X-ray absorption line spectrum recorded so far (Fig. 11.3). Many lines from several ionisation stages of Ne and Mg can be seen and high abundances were derived: Ne = Mg = 2% by mass. It is speculated that H1504+65 is a naked C-O white dwarf or even a WD with a O-Ne-Mg core resulting from carbon burning .
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