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There are 7 planetary nebulae in which the central stars are sdO stars (Werner et al., 1991, A$\&$A 244,437 ). Searches for old nebulae around known hot sdO stars (= PG 1159 type stars), however, have been unsuccessful (M\'endez et al. 1988, A$\&$A 198, 287, Kwitter et al., 1989, A.J. 97, 1423). These negative results suggested that either these sample stars had not passed through the PN phase, or that their nebulae had sufficient time to disperse (Kwitter et al., 1989).
We imaged the field around the non-variable, hot sdO star PG 1520+525 with the KPNO 0.9m telescope and T2KA CCD for 30 minutes. A spherical nebula centered on the white dwarf is easily seen in this H$\alpha$+[NII] image and has a diameter of 11.1 arcminutes. Given a distance of 1.1 kpc and an average expansion velocity of 20 km-s{$^{-1}$}, this diameter suggests a kinematic age for the nebula of 87,000 years. This age estimate combined with the central star temperature of 140,000 K is in agreement with PG 1520+525 having a central star mass of about 0.8 M{$_o$} (Sch\"onberner, 1993, IAU 155 Planetary Nebulae p. 420). This high central star mass is in agreement with the suggestion that massive stars of type WC are the progenitors of the hot sdO stars (Werner et al., 1991).
Spectra of the brightest part of the nebula, where it seems to interact with the ambient medium, were taken with the KPNO 2.1m telescope and Goldcam. The [OIII] (5007A and 4959A) lines are clearly visible. Subsequently, we reviewed the photographic spectra taken of this object by Kwitter et al. (1989). In fact, the [OIII] lines are just visible on their plate of PG 1520+525 kindly loaned to us by Phil Massey. We then checked the remainder of the plates collected in that study, but found no other strong case for the presence of a nebula around any of the other sdO stars observed in their survey. The planetary nebulae from these stars are probably of too low surface brightness, having faded below the detection threshold of their survey.
