Gas phase carbon abundances have been determined for a number of bright planetary nebulae from new photoelectric measurements of optical recombination lines (C II λ 4267, C III λ 4650 and C IV λ 4659). Because of blending problems for the latter two features, the abundance for any object may have substantial errors, but the average abundance should be reliable. For the twelve best observed planetaries, this average is C/H = (8.4 ± 2.9 s.d.) x 10−4 by number. If it is assumed that the planetary progenitors had essentially solar abundances, then, based on Cameron's most recent results (C/H = 4.2 × 10−4, O/H = 6.9 × 10−4), it appears that the planetary ejecta have been enriched in carbon, presumably because of dredging of newly synthesized triple α carbon by helium shell flashes during the late evolution of the progenitor. The helium abundance is also slightly high, as would be expected in this interpretation. Since the mean planetary carbon abundance exceeds the solar oxygen abundance, it is possible that the progenitor became a carbon star prior to the ejection of the planetary; that may even have caused the ejection. Because the planetaries in this study were drawn from Peimbert's samples with relatively low mass progenitors (1 – 2.5 M⊙; these are not significantly helium- and nitrogen-rich objects), such a process might be a general feature of late double shell source evolution.

The determination of the chemical abundances in planetary nebulae and HII regions from the observed intensities of emission lines usually needs a correction for the unseen ionization stages. An empirical correction was first introduced by Peimbert and Costero (1969) as a factor ICF (ionization correction factor) which was obtained assuming that ions with similar ionization potentials are equally populated. Recently, Natta et al. (1980) discussed the problem of sulphur abundance in nebulae and proposed an ICF for sulphur which takes into account the radiation field as well as the ionization equilibrium of 0+, 0++, S++ and S+3. However they did not consider the effect of the charge exchange reactions which can be very important in determining the ionization equilibrium of the gas (Pequignot et al., 1978).

New observations of the unidentified 30 μm spectral feature found in two planetary nebulae and four carbon stars are reported. The carbon star IRC + 10216 was observed from the Kuiper Airborne Observatory over the range 30 to 37 μm. These new data confirm the existence of the feature and determine its shape beyond 30 μm. The determination of the long wavelength end of the spectral excess depends critically on the assumed underlying continuum. A combined spectrum of the excess is presented and shows the feature to extend at least to 37 μm.

The OH maser emission from OH/IR stars (type IIb) originates from the thick circumstellar dust shell. As the envelope is expanding and the strongest maser emission comes from the front- and the backside, where the gain pathlength is longest, we can measure a definite phaselag (and hence a diameter) between front- and backside of the shell for all sources that are variable.

From observations we find that the OH luminosity LOH of an OH/IR star increases with R2, where R is the size of the masing region. From this correlation we deduce that the mass loss rate M, the expansion velocity ve and LOH are related by LOH ~(M/ve)2. Next we consider the large range that is observed in LOH and the steep OH luminosity distribution for OH/IR stars. Both facts can be explained by the postulate that these objects undergo accelerated mass loss, and thus steadily increase their OH luminosity. We propose that OH/IR stars are at the extreme end of the Asymptotic Giant Branch and that many of them are in the process of blowing off their entire envelope in a superwind phase. Their mass loss rate during this superwind, as deduced from OH observations of the circumstellar shell, is given by a simple modification of the Reimers equation. This modification connects the superwind continuously to the Reimers wind and it provides observational evidence for the formation of a planetary nebula.

The catalogue contains 460 nuclei of 393 true and 67 possible planetary nebulae; 87 of these were discovered after the publication in 1967 of the catalogue of planetary nebulae of Perek and Kohoutek.

B and V magnitudes for the central stars of a number of planetary nebulae are presented. The observations were obtained between 1971 and 1981 with the University of Illinois one-meter telescope at Prairie Observatory. The average magnitudes presented are accurate extractions of the stellar continuum flux from the total (stellar plus nebular) measured flux (see Kaler, 1976, Astrophys. J., 210, 113).

The UBV- and spectral observations of several variable planetary nebulae were continued at the Crimean Station of Sternberg Astronomical Institute.

Recently Pottasch (1981, Astron. Astrophys. 94, L13) published extremely high effective temperatures of some central stars of planetary nebulae (> 200 000 K). Our study of planetary nebulae based on photoelectric photometry does not confirm his results. A histogram of Tz(HI) and Tz(HeII) shows smooth distribution of Tz with the maximum of about 48 000°K (HI) and 90 000°K (HeII), respectively; the effective temperature of none of the 62 planetary nuclei exceeds 120 000°K.

The flux originating from the central stars of 27 planetary nebulae in the spectral range 1200-2000 Å has been deduced from the analysis of a large number of released IUE low resolution spectra.

A spectrophotometric survey has been made for about 20 central stars of planetary nebulae, with emphasis mainly on hot stars. We use low-resolution IUE spectra, observed by ourselves or obtained from Data Center, together with, in some cases, results from optical observations. Data have been extracted and merged, regions of saturation eliminated, ITF errors corrected and nebular continua subtracted. Careful assessments have been made of reddening constants, and of data used to calculate Zanstra temperatures.

Ultraviolet and optical observations of seven very low excitation nebulae (He 2-131, He 2-138, TC 1, M 1-11, M 1-12, M 1-26, H 2-1) and of two low-excitation nebulae (IC 418 and He 2-108) are discussed.

We have obtained ultraviolet and optical spectrophotometry for two PN in the SMC (N87, L302) and for six in the LMC (N28, N66, N97, N141, N201, N203). The data were obtained with the IUE (eight nebulae), the 3.9 m AAT (six nebulae) and the 1.9 m SAAO reflector (two nebulae). Nebular temperatures, densities and abundances are presented. The nebular continua were calculated and subtracted from the observed continua, allowing the central star energy distributions and hydrogen and helium Zanstra temperatures to be derived. Our results for the effective temperatures and absolute magnitudes and luminosities of the central stars are presented, including data on two Wolf-Rayet central stars (WC5 and WC8) and one Of central star.

We have observed IC 4642 with the AAT and the IUE. It is of exceptionally high-excitation class, as judged by ratios such as He II/Hβ and (Ne V)/He II.

Individual distances to planetary nebulae (PN) which are independent of any assumption of average nebular characteristics, can be found if one knows the relation between interstellar extinction (E(B-V)) and distance along the line-of-sight to the PN, together with the E(B-V) towards the PN itself (Lutz, 1973 and Acker, 1978). We used VBLUW-photometry (Lub and Pel, 1977) to derive accurate E(B-V)'s and distances of stars up to V-magnitude + 14, within 0°.3 from the PN. Table 1 lists the PN we studied. The E(B-V)'s of the PN are derived from:

Most distances of planetary nebulae are known roughly within a factor of 2 or larger, except for some special objects for which the uncertainty can be as low as 50%. In the present work both IUE interstellar Lyman alpha profiles and 21 cm HI line surveys are used to infer the distances of four planetary nebulae.

HI observations at 21 cm have been made with the Westerbork Synthesis Radio Telescope in the direction of six planetary nebulae located in or near the galactic plane (N 7027, 2440, 6537, 6572, 7026, 7354).

We have detected 34 OH/IR stars within 1 degree of the galactic centre by their OH emission line at 1612 MHz (18 cm) using the Effelsberg 100 m telescope and the Very Large Array. The spatial distribution and the distribution of the radial velocities show that practically all stars are within 150 pc from the Galactic centre, and that the number of foreground objects is very small. The projected distribution of the stars is similar to that of the surface brightness at 2.4 μm. Since the 2.4 μm radiation is supposed to be due to red giants, the OH/IR stars are probably members of the same population. The stars have considerable random velocities (velocity dispersion in one coordinate of 150 ± 50 km s−1), but show general Galactic rotation. The high velocity dispersion is remarkable for objects of this population.

We used a sequence of velocity-modulated photographs to find and measure the radial velocities of faint planetary nebulae in the center of M31. The photographs were made with a Velocity Modulating Camera (VMC) which consists of a temperature-tuned 2.1 Å (FWHM) (O III) λ 5007 interference filter, a cooled, two-stage image intensifier, and a calibrating photomultiplier. The camera was mounted at the Cassegrain focus of the Shane 3 m telescope at Lick Observatory. We identified 19 new planetary nebulae, bringing the total number of known planetaries within 250 pc of M31's nucleus to 45. From the plate series, we derived radial velocities and relative brightnesses from 32 of the nebulae and placed radial velocity limits on the remaining nebulae in the field. By applying the method of maximum likelihood to the observed radial velocity distribution, we derive a mean heliocentric velocity of −309 (±25) km s−1 and a velocity dispersion of 155 (±22) km s−1 for the planetary nebulae.

A low surface brightness planetary nebula with weak ring structure and a diameter 22 arcsec has been discovered in the southwest edge of the open cluster NGC 6067. A calibrated spectrum shows strong He II 4686 (80% Hβ) and an Hα/Hβ ratio of 5.8. The Hα/Hβ ratio yields c = 0.88 which corresponds to Av = 1.88. This is significantly greater than Av for the cluster (1.17) and leads to a distance for the nebula of 3370 pc assuming extinction to be uniform with distance. This leads to a radius of 0.18 pc and a mass of 0.07 solar masses (assuming ε = 0.5).