A number of objects identified in the Palomar-Green survey of faint blue stars were classified sdO(D) (Green et al. 1986), having strong HeI lines and weak or absent Balmer lines. Similar objects have been identified in other surveys of faint blue stars (Edinburgh-Cape, Hamburg-Schmidt) and are alternatively known as helium-rich subdwarf B stars (He-sdB) (Moehler et al. 1990). In an attempt to clarify the classifications currently in use, Drilling (1996) introduced the term sdB4 to denote these objects. A list complete to December 1995 was compiled by Jeffery et al. (1996).
Apart from spectroscopic identification the only
previous work on these stars is a preliminary study which
demonstrated the prototype PG1544+488 to be helium
enriched (Heber et al. 1988) and analyses of JL87
(Schulz et al. 1991), LSIV
(Viton et al.
1991) and LB1766 (Kilkenny & Busse 1992).
These objects were shown to have
similar 
to the more numerous sdB stars, but
slightly lower surface gravity and much higher helium
abundances.
The existence of extremely helium-rich stars
otherwise similar to the better known but extremely
hydrogen-rich subdwarf B stars suggests that they
represent a vital link in the late stages of stellar
evolution. A number of possibilities arise. Are they
intrinsically the same objects as normal sdBs, but with
their surface hydrogen mixed into helium-rich layers just
below the surface? Are they more closely related to the
hotter helium-rich sdOs lying close to the helium main-
sequence and beyond the blue end of the horizontal
branch? Or are they more closely related to the extreme
helium stars, particularly the hottest high-gravity
members of the class such as LSS3184 (Drilling et al. 1997)
and LSIV
(Jeffery 1996)?
In order to ascertain the nature of our targets more precisely, the authors have obtained optical spectroscopy and ultraviolet spectrophotometry of a significant sample. In this paper we examine the optical spectroscopy and review the classification of the targets. Refining the classification is a first step towards making more detailed atmospheric analyses of individual objects.
| Object | m B | Instrument | Date |
| PG0902+058 | 14.1 | WHT+UES | 1995 May 6, 7, 8 |
| PG0902+058 | CA | 1988 May 25 | |
| PG0909+276 | 10.7 | WHT+UES | 1995 May 6, 7, 8 |
| PG0909+276 | WHT+ISIS | 1996 May 30 | |
| PG0921+311 | 14.4 | WHT+ISIS | 1996 May 30 |
| HS1000+4704 | 17.1 | WHT+ISIS | 1996 May 31 |
| PG1127+019 | 13.0 | WHT+UES | 1995 May 6, 7 |
| PG1127+019 | WHT+ISIS | 1996 May 31 | |
| PG1127+019 | CA | 1988 May 26 | |
| PG1230+067 | 12.3 | WHT+UES | 1995 May 6, 7, 8 |
| TON107 | 16.3 | WHT+ISIS | 1996 May 30 |
| PG1413+113 | 15.6 | CA | 1988 May 25 |
| PG1415+492 | 14.4 | WHT+UES | 1995 May 7, 8 |
| PG1415+492 | CA | 1988 May 26 | |
| PG1437+727 | 13.6 | WHT+UES | 1995 May 7 |
| PG1437+727 | WHT+ISIS | 1996 May 30 | |
| PG1441+407 | 15.5 | WHT+ISIS | 1996 May 31 |
| PG1526+440 | 15.7 | WHT+ISIS | 1996 May 31 |
| PG1544+488 | 12.0 | WHT+UES | 1995 May 7, 8 |
| PG1544+488 | CA | 1988 May 26 | |
| PG1552+464 | 16.0 | WHT+ISIS | 1996 May 31 |
| PG1554+408 | 15.9 | WHT+UES | 1995 May 9 |
| PG1554+408 | WHT+ISIS | 1996 May 31 | |
| PG1559+222 | 14.5 | WHT+UES | 1995 May 7, 8 |
| PG1559+222 | CA | 1988 May 26 | |
| PG1600+171 | 16.3 | WHT+ISIS | 1996 Jun. 1 |
| PG1607+173 | 10.2 | WHT+ISIS | 1996 May 30 |
| PG1615+413 | 16.4 | WHT+ISIS | 1996 Jun. 1 |
| PG1629+466 | 14.0 | CA | 1988 May 27 |
| PG1629+466 | WHT+UES | 1995 May 9 | |
| PG1648+315 | 16.0 | WHT+ISIS | 1996 Jun. 1 |
| PG1658+273 | 15.7 | WHT+ISIS | 1996 Jun. 1 |
| PG1715+273 | 16.2 | WHT+ISIS | 1996 Jun. 1 |
| HS1843+6343 | 16.1 | WHT+ISIS | 1996 May 30 |
| LS IV-14 116 | 13.0 | WHT+ISIS | 1996 May 30 |
| PG2128+096 | 14.3 | WHT+UES | 1995 May 9 |
| PG2128+096 | WHT+ISIS | 1996 May 30 | |
| PG2215+151 | 13.9 | WHT+ISIS | 1996 Jun. 1 |
wrong star of close pair 
Å only.