Nine helium rich sdO stars are found to show no trace of hydrogen on high resolution visual spectra. Effective temperatures derived from UV fluxes range from 42500 K to 80000 K. A dichotomy with respect to the C/N ratio is found which is reminiscent of the OBN and OBC stars near the main sequence. It is estimated that about 20% of the sdO stars are extremely helium rich. This fraction compares nicely with those of the (helium rich) DB white dwarfs (20%) and the helium rich central stars of planetary nebulae (less than 35%).

Three extremely helium-rich sdO stars (LSE 153, LSE 259 and LSE 263) were analyzed spectroscopically by means of detailed NLTE model atmospheres. These stars are very hot, with effective temperatures ranging from 70000 to 75000 K and gravities between log g = 4.4 and 4.9. Upper limits for the hydrogen abundance were also derived. The evolutionary status of the sdO stars is discussed and it is concluded that they evolve from the asymptotic giant branch towards the white dwarf stage. A possible evolutionary link between these hot stars and the extreme helium stars of spectral type B is discussed.

New spectra of A78 and A58 at different positions in the nebulae are presented. An abundance gradient is found in A78, extending quite close to the center. Similarly the nebulous knot near the center of A58 has considerably higher heavy element abundances than the outer regions of this nebula. The ionization state is considerably lower in A58 than A78. In A78 most of the neon is in the form of Ne+3 and Ne+4, indicating that the standard ionization correction factor as used by Jacoby and Ford, is substantially in error. Finally, the very high infrared excesses found in this nebulae are discussed.

White dwarfs with helium-rich atmospheres constitute about one fifth of the white dwarfs hotter than 12,000 K. They appear to have a mass distribution similar to the hydrogen atmosphere (DA) stars, and are similar in other properties. However, the temperature distribution exhibits a deficiency of DB/DO stars in the interval 25,000-45,000 K, which implies evolution in the dominant surface composition as the stars cool. The hottest group of transition DO white dwarfs are the pulsating objects of the PG1159 class. The central star of Kl-16 is a related object, as may be the newly discovered very hot star H1504+65, which shows no detected surface features of either hydrogen or helium.

Ultraviolet energy distributions are analyzed for several hot, helium atmosphere DB white dwarfs, including the four known pulsating stars which define an empirical DB instability strip. Temperatures are derived exclusively from fits to the ultraviolet energy distributions. The blue edge of the empirical DB instability strip lies at 30,000 ± 4,000 K, and the red edge lies near 24,000 ± 2,000 K. The hottest DB star — and the only known one hotter than the instability strip — is PG0112+104 at or above 30,000 K. This leaves no known helium-atmosphere degenerate stars in the interval 30,000 ≤ Te ≤ 45.000K.

For white dwarfs with effective temperatures smaller than 12000 K, the percentage of objects with a helium-rich atmosphere increases compared to the hydrogen-rich sequence. The carbon abundance, which can be determined from line and band strengths (Bues, 1973; Koester et al., 1982), varies by more than a factor of 1000 within this class. Moreover, for the subclass of white dwarfs with strong magnetic fields, the abundance ratio of H/He differs from that of the DB and DA sequences. The hot star Feige 7, analyzed by Liebert et al. (1977), shows lines of hydrogen and helium at a comparable strength for a moderately strong field of 103 Tesla. If there is any chance of finding white dwarfs which are descendants of hot, non-degenerate helium stars with rotation and magnetic fields, then it should be within these objects of mixed composition.

We report fast photometric observations on AM CANUM VENATICORUM (AM CVn) the ultra short period, hydrogen deficient variable. We have detected on 24th February, 1985 an intense flare of (Δm)peak≈0.34 in white light lasting over 200s. Following this flare we observe an enhanced double humped structure lasting for 1051s which is the dominant periodicity exhibited by AM CVn. We have also detected the 525s and 1051s periods. In addition, we report flickerings, lasting typically 1-2 minutes, that are characteristic of cataclysmic variables.

The Infra-Red Astronomical Satellite, working at 12μm, 25μm, 60μm and 100μm, has observed stars in several subgroups of the hydrogen deficient stars. Observations of most known R CrB stars are reported here, as well as data for several HdC stars and other carbon-rich stars, together with some intermediate helium rich stars and related objects. Also given are LRS spectra of R CrB, RY Sgr, υ Sgr and some Carbon stars. Initial reductions of the additional observations from IRAS show an extended dust shell around R CrB, and probably also around SU Tau.

A review is given on the properties of the pulsating hydrogen-deficient stars. Since they lie in a large area of the HR diagram, the properties of pulsations are greatly different among them. Pulsations tend to change from radial pulsations to non-radial pulsations as the surface gravity increases. Strong nonadiabaticity affects the dynamical properties of pulsations in high luminosity helium stars. The kappa and gamma mechanisms for helium ionization excite radial pulsations in relatively cool helium stars and nonradial pulsations in DB (helium atmosphere) white dwarfs, while the cyclical ionization of the K-shell electrons of carbon and oxygen seems to be responsible for the excitation of the nonradial pulsations in the GW Vir (PG1159-035) stars (very hot hydrogen-deficient pre-white dwarf stars). The excitation mechanism of the radial pulsations in a unique extreme helium star V652 Her (BD+13°3224) is not known. Period changes have been detected for many pulsating helium stars except the pulsating DB white dwarfs.

Application of the homogeneous nucleation theory to the problem of R CrB stars shows that the radial distance of the inner boundary of the carbon supersaturation region is at about 12 photospheric radii for a stellar effective temperature Te = 6000 K. Formation of an optically thick dust shell becomes possible at mass loss rates Ṁ ≳ 10−6 M⊙/yr. However, the upper limit of this mass loss rate cannot considerably exceed 10−5 M⊙/yr since, at higher Ṁ, the theoretically predicted rate of the visual brightness decline is larger than that derived from observations. Comparison of the theoretically predicted radii of dust grains with those observed in R CrB and RY Sgr shows that the mass loss rate in these stars should be in the range of 1 × 10−7 M⊙/yr to 3 × 10−6 M⊙/yr.

In the absence of mass loss, diffusion leads to underabundances of He in main sequence stars. Because of a very strong observational link with Ap and He weak stars, it has however been suggested that diffusion is the explanation for the He rich stars of the upper main sequence. This requires a mass loss rate of 10−12 Mo yr−1 or slightly lower. The mass loss rate must decrease as Teff increases. Magnetic fields must apparently be involved to reduce the mass loss rate. Since this model predicts that the CNO abundances should be normal in the cooler He rich stars, it leads to a clear observational test. Detailed calculations should be made to confirm the importance of this test. The effects of separation in the wind, the atmosphere and the envelope are discussed to conclude that separation in the atmosphere is likely to be most important. The importance of diffusion for He rich white dwarfs and horizontal branch stars are briefly discussed.

Our present knowledge about the evolutionary status of extremely hydrogen-deficient stars is reviewed. Possible schemes for their creation are discussed, with special emphasis on a recently proposed binary scenario, according to which they are the result of the merging of two white dwarfs. Finally, the possible generic links between the different groups of extremely hydrogen-deficient stars are briefly discussed.

The modem theory of stellar evolution leads to several possible scenaria for the formation of many types of hydrogen deficient (usually helium rich) stars. Some of these scenaria are briefly discussed in this paper to provide possible explanations for origins of R Cr B, hot helium and Wolf-Rayet stars; hot sub dwarfs, non-DA degenerate dwarfs and helium stars in binaries like KS Per. The merger of two degenerate stars in close binaries due to the radiation of gravitational waves can lead to several types of helium and carbon rich stars as well as to supernova explosions.

This has been the first international conference devoted to hydrogen-deficient stars and related objects. As Professor Hunger stated in his introduction, one aim of a meeting such as this is to review our achievements and find out what conclusions we can reach about the nature and origin of the objects we study. However, it is even more important that we identity the questions that we must ask and particularly which questions it may be within our means to answer. A newspaper reporter is said to have asked the Chairman of our Local Organizing Committee what we proposed doing to correct this hydrogen deficiency!

Our meeting started with Drilling’s review of the galactic distributions and velocity dispersions of the various groups of hydrogen-deficient stars. They cover a wide range from the Population I rapidly rotating intermediate helium stars to the extreme helium stars and cool hydrogen-deficient stars, including the R Coronae Borealis (RCB) variables, which show characteristics of Intermediate Population II. We certainly need to be very clear how each of these classes is defined, particularly as there are a few objects, very definitely hydrogen-deficient, which fail to fit neatly into any of our categories. One star which has been the subject of three contributed papers and mentioned in others has too much hydrogen and too much gravity to be an extreme helium star, too little hydrogen to be an intermediate and too little gravity to be a subdwarf. I refer to BD+13°3224, otherwise known as V652 Her. As many of these stars are discovered to be variable they will be allotted variable star names. To avoid confusion it seems best to mention all known names.

The following four tables were originally presented as part of the paper entitled ‘Basic Data on Hydrogen-Deficient Stars’ by J. S. Drilling, which appears earlier in this volume. A number of corrections and additions have been made by the participants, mostly by P. W. Hill using the SIMBAD data base. A much improved version of the catalog therefore follows. Helium-rich central stars of planetary nebulae, helium-rich white dwarfs, and Wolf-Rayet stars are not included. A complete list of helium-rich central stars is given by Mendez et al. elsewhere in this volume.