  We report initial results from an ongoing spectroscopic survey of central
stars of faint planetary nebulae (PNe), obtained with the Low-Resolution
Spectrograph on the Hobby-Eberly Telescope. The six PN nuclei (PNNi) discussed
here all have strong emission at the O VI 3811-3834 A doublet, indicative of
very high temperatures. Five of them--the nuclei of Ou 2, Kn 61, Kn 15, Abell
72, and Kn 130--belong to the hydrogen-deficient PG 1159 class, showing a
strong absorption feature of He II and C IV at 4650-4690 A. Based on
exploratory comparisons with synthetic model-atmosphere spectra, and the
presence of Ne VIII emission lines, we estimate them to have effective
temperatures of order 170,000 K. The central star of Kn 15 has a
Wolf-Rayet-like spectrum, with strong and broad emission lines of He II, C IV,
N V, and O V-VI. We classify it [WO2], but we note that the N V 4604-4620 A
emission doublet is extremely strong, indicating a relatively high nitrogen
abundance. Several of the emission lines in Kn 15 vary in equivalent width by
factors as large as 1.5 among our four observations from 2019 to 2022, implying
significant variations in the stellar mass-loss rate. We encourage
spectroscopic monitoring. Follow-up high-time-resolution photometry of these
stars would be of interest, given the large fraction of pulsating variables
seen among PG 1159 and [WO] PNNi.
