The chairman has kindly encouraged, not to say entreated, me to write a few remarks concerning the subject of your symposium, which I was unfortunately unable to attend. This I am happy to do: the hydrogen-deficient stars are dear to my heart and even though I haven’t contributed anything to the subject for several years, it is certainly nice to be remembered. From an outsider, then, a few thoughts.

To quote from Miss Payne, in her classical study of 1925: The uniformity of composition of stellar atmospheres appears to be an established fact.” Certainly for the time that statement was beyond reproach. Yet even then the seeds of hydrogen deficiency had already been sown. Mrs. Fleming, in noting the presence of bright Hβ in u Sagittarii, in 1891, further states that its spectrum “is remarkable, since the hydrogen lines are very faint and of the same intensity as the additional dark lines.” Further, Ludendorff, in a paper written on Aug. 16, 1906, discovered the complete absence of the Hγ line in R Coronae Borealis (a similar situation with respect to Hβ and Hδ being confirmed by Frost). And by a remarkable coincidence, a Harvard objective-prism plate taken the very same day was described by Miss Cannon as showing very little absorption at the G band. Both HD 30353 and RY Sagittarii are stated in the Henry Draper Catalogue to show a spectral resemblance to R CrB. And finally, the non-typical weakness of the G band of the carbon star HD 182040 was pointed out by Rufus as early as 1923.

The current state of our knowledge on the distribution and motions of various types of hydrogen-deficient stars, and of their positions in the H-R diagram, is reviewed. It is concluded that the extreme helium stars (with the exception of the H-deficient binaries) and cool hydrogen-deficient stars belong to the population of the Galactic nuclear bulge, whereas the intermediate helium stars are young stars of Population I. The helium-rich sdO stars appear to be a local sample which is predominantly Population I.

Radial velocities have been determined for nearly all the extreme helium stars and for the very hot sdO stars discovered by Drilling from high-resolution spectra obtained at ESO and at CTIO.

Spectroscopic fine analyses of hot extreme helium stars are reviewed. The chemical composition is discussed in detail and conclusions as to the nuclear history of the atmospheric material are drawn. Evidence of inhomogeneities among the hot extreme helium stars is presented and it is concluded that the extreme helium stars can be found in the halo population (BD+10°2179) as well as in the disc population. Their mass loss rates are of the same order of magnitude (10−9 M⊙/yr) as those of normal stars. Four helium rich sdO stars are identified as possible descendants of the B-type extreme helium stars.

The purpose of this paper is to review the photometric histories of the extreme helium stars.

High resolution spectra of six newly discovered extreme helium stars are presented. LSS 5121 is shown to be a spectroscopical twin of the hot extreme helium star HD 160641. A preliminary LTE analysis of LSS 3184 yielded an effective temperature of 22000 K and a surface gravity of log g = 3.2. Four stars form a new subgroup, classified by sharp-lined He I spectra and pronounced 0 II spectra, and it is conjectured that these lie close to the Eddington limit. The whole group of extreme helium stars apparently is inhomogenous with respect to luminosity to mass ratio and chemical composition.

The occurrence of emission lines in high resolution blue spectra of four extreme helium stars is reported. This phenomenon is most apparent in the case of BD−9°4395 which displays emissions in He I, λ 3889 Å, C II, λ 4267 Å and in the C II multiplet No. 1 (λ 4740 Å). The latter is also present in LSE 78 and found to be variable. The C II emission lines are regarded as evidence for extended moving envelopes around BD−9°4395 and LSE 78. A possible relation to the hot R CrB star V348 Sgr is discussed.

HD 160641 and LSS 5121 show unidentified emission lines (λλ4485.6 Å, 4504.0 Å) which are also observed in some Of stars. No emission lines were found in the blue spectra of 12 extreme helium stars.

The spectrum of the extreme helium star BD−9°4395 has already been noted for emission lines of Hel 3889Å and CII 4267Å. Further anomalies in the Hel and CII spectra have been observed, including the identification of additional emission lines, asymmetry in the He I line profiles and variability in all features. It is suggested that these features may be associated with other evidence for non-radial pulsations in BD−9°4395.

Simultaneous radial velocity and photometric observations are reported for the variable extreme helium star HD 160641. Pulsation in the 1 - 4 mode could explain the observed variations; the corresponding Wesselink radius would be 8 ± 2 R⊙. The consequent luminosity of log (L/L⊙) - 4.8 ± 0.2 would be consistent with Schonberner's evolutionary model for a lM⊙ extreme helium star.

Photometry of the variable extreme helium star BD+1°4381 is used to improve the 21.2 day period found by Jeffery & Malaney (1985). The amplitude of the light-curve appears to be variable, resembling that of RY Sgr. Variability in the extreme helium star BD−1°3438 is also reported.

A preliminary fine analysis of the atmosphere of the pulsating extreme star V652 Her (=BD+13°3224) is reported. The mean effective temperature and temperature variation have been determined from the ionisation equilibria of Si and N. Both of these elements are strongly overabundant, whilst C appears to be underabundant.

New radial velocity data for the pulsating extreme helium star V652 Her (BD+13°3224) have been obtained with a time resolution of 100 s. High frequency structure in the radial velocity curve is detected, and a comparison with previous data suggests that the detailed shape of the velocity curve is variable. The data imply that the effective surface gravity must increase by a factor of 4 at minimum radius.

Further optical and infrared (J-filter) observations are presented for the extreme helium star BD +13°3224. No infrared excess, which could have been attributed to a cool companion, is found. Very recent times of maximum light confirm the ephemeris cubic term already proposed. Current models for the evolution of BD +13°3224, while approximately accounting for the period decrease rate (Ṗ), do not explain the decreasing Ṗ indicated by the cubic term.

The chemical composition of the R Coronae Borealis and cool hydrogen deficient carbon stars is reviewed. Similarities and differences between these stars and the hot He stars are noted. Proposed origins for the hydrogen deficient stars are sketched. Recent claims that normal (spectral type N) cool carbon stars are hydrogen deficient are shown to be unfounded. Attention is drawn to the curious case of pop. II variables (RV Tauri, W Virginis, and RR Lyrae stars) whose atmospheres show striking deficiencies of heavy elements and may be hydrogen deficient.

RCB stars are surrounded by circumstellar dust and gas moving radially outwards at ~200 km/sec. The circumstellar shell is made up of discrete puffs of matter, a typical puff occupying an area ~0.03 of a complete shell. On the average puffs are ejected about once every 40 days (comparable with the known pulsation periods of RCB stars). The reddening law of the dust indicates that it is composed of small carbon particles (radii ~100A). The flux from the shell at L typically varies by 1 to 3 mags over periods of 1000-2000 days. The average mass loss rate is ~10−6MO/yr.

Polarimetric observations of HD 30353, SU Tau, XX Cam, R Cr B,UV Cas, BD+13 3224,BD+10 2179 and HD 124448 are presented. The linear polarization of HD 30353 is found to vary appreciably at Hα over time scale as short as one day. It is also found that SU Tau shows significant variation in polarisation even at light maximum.

Deep light minima of R CrB type stars are considered to occur due to the carbon abundance excess in the atmospheres of these stars, and condensation of carbon into graphite dust leads to light decrease (Zhilyaev et al., 1978). The cause and conditions of dust formation are not clear. Therefore, it is necessary to find out regularities in the occuring of light minima and to search for correlations between different parameters of this type stars. This paper deals with the time intervals between the series of consecutive light minima.

A long series of observations of the R Coronae Borealis variables brightness allowed to study repeatedly their light curves in order to obtain their periodicity or other parameters in occuring of deep light minima. The work by Sterne (1935) was the first, in which the conclusion was made that R CrB is an irregular variable. Next investigations contained the analyses of light curves and calculations of the number of minima planned in the definite intervals, f.ex., 0-300, 300-600 days etc. (Howarth, 1977; 1978). Other papers (Tempesti et al., 1975) considered the duration of minima and maxima states. In all these investigations the time was counted off from the moment when a star reached a definite light decrease magnitude, f.ex., Δ m = 1m. The conclusions obtained were similar: the considered parameters were in agreement with Poisson statistics.