CrB Stars Research Articles

An Episode of Occultation Events in Gaia21bcv

A previously unremarkable star near the Canis Major OB1/R1 association underwent an episode of multiple deep brightness minima. Light curves based on archival Gaia, Zwicky Transient Facility (ZTF), and NEOWISE data and additional observations from the Las Cumbres Observatory and UKIRT show that the star was not variable prior to 2019 August 18 (MJD 58700), and on that date started showing brightness dips of up to 3 mag in the Gaia G and ZTF r bandpasses. After MJD 59500, ≈800 days after the onset of these dipping events, the star returned to its previous brightness, and no significant dipping events have been recorded since. Compared to the stable phase, NEOWISE IR photometry in the W1 and W2 bands indicates a generally redder color, and both decreases and increases in brightness at different times during the dipping episode. The spectrum of Gaia21bcv taken after the end of the dipping episode shows several neutral and ionized metal absorption lines, including Li, indicating a spectral type of ≈K5. Variable emission from [O i] was observed. The Hα absorption in Gaia21bcv is too faint and irregular for this spectral type, indicating that the line is partly filled in by variable emission, a signature of weak episodic accretion. Gaia21bcv lies above the zero-age main sequence, but is much fainter than typical R CrB stars. We interpret the light curve of Gaia21bcv as being similar to the occultation events in ϵ Aurigae, i.e., occultation by a disk around a companion object orbiting the primary star.

Orbital Decay in an Accreting and Eclipsing 13.7 Minute Orbital Period Binary with a Luminous Donor

We report the discovery of ZTF J0127+5258, a compact mass-transferring binary with an orbital period of 13.7 minutes. The system contains a white dwarf accretor, which likely originated as a post–common envelope carbon–oxygen (CO) white dwarf, and a warm donor (T eff,donor = 16,400 ± 1000 K). The donor probably formed during a common envelope phase between the CO white dwarf and an evolving giant that left behind a helium star or white dwarf in a close orbit with the CO white dwarf. We measure gravitational wave–driven orbital inspiral with ∼51σ significance, which yields a joint constraint on the component masses and mass transfer rate. While the accretion disk in the system is dominated by ionized helium emission, the donor exhibits a mixture of hydrogen and helium absorption lines. Phase-resolved spectroscopy yields a donor radial velocity semiamplitude of 771 ± 27 km s−1, and high-speed photometry reveals that the system is eclipsing. We detect a Chandra X-ray counterpart with L X ∼ 3 × 1031 erg s−1. Depending on the mass transfer rate, the system will likely either evolve into a stably mass-transferring helium cataclysmic variable, merge to become an R CrB star, or explode as a Type Ia supernova in the next million years. We predict that the Laser Space Interferometer Antenna (LISA) will detect the source with a signal-to-noise ratio of 24 ± 6 after 4 yr of observations. The system is the first LISA-loud mass-transferring binary with an intrinsically luminous donor, a class of sources that provide the opportunity to leverage the synergy between optical and infrared time domain surveys, X-ray facilities, and gravitational-wave observatories to probe general relativity, accretion physics, and binary evolution.

Ton S 415: a close binary containing an intermediate helium subdwarf discovered with SALT and TESS

ABSTRACT Hot subdwarfs are early-type low-mass stars lying between the main sequence and the white dwarf sequence. The majority have helium-burning cores and helium-poor surfaces. A minority have helium-rich surfaces. A few, including Ton S 415, have surfaces with between 10 per cent and 90 per cent helium (by number). Their properties are diverse and their origins mostly unknown. Ton S 415 was identified as a suspected binary in a Southern African Large Telescope (SALT) survey of hot subdwarfs from its large radial velocity. Using SALT spectroscopy and Transiting Exoplanet Survey Satellite light curves, we confirm that Ton S 415 is a close binary with an orbital period $p=84.6460\pm 0.0004\, {\rm min}$ and velocity semi-amplitude $K=175.5\pm 1.0\, {\rm km\, s^{-1}}$. Analysis of the SALT spectrum and broad-band spectral energy distribution shows the visible star to have an effective temperature $T_{\rm eff}=43\, 300\pm 1000\, {\rm K}$, surface gravity $\log g/{\rm cm\, s^{-2}}=5.89\pm 0.10$, surface helium-to-hydrogen ratio log y = −0.62 ± 0.10, radius $R=0.1074\pm 0.0025\, R_{\odot }$, and mass $M=0.33\pm 0.09\, {\rm {\rm M}_{\odot }}$. By modelling the light curve, we estimate the binary mass ratio to be q = 0.7 ± 0.3 and infer an unseen white dwarf companion with a mass of $M_{\rm WD}=0.47\pm 0.24\, {\rm {\rm M}_{\odot }}$. The results are consistent with a post-common-envelope binary that lost its hydrogen envelope before reaching the peak of the red giant branch, becoming a hot subdwarf with a non-canonical mass. We predict that the system will evolve into a double white dwarf binary before eventually either forming a stably accreting AM CVn system, or merging to form an R CrB star.

Formation of ultra-massive carbon–oxygen white dwarfs from the merger of carbon–oxygen and helium white dwarf pairs

ABSTRACT Ultra-massive white dwarfs (UMWDs) with masses larger than 1.05M⊙ are basically believed to harbour oxygen–neon (ONe) cores. Recently, Gaia data have revealed an enhancement of UMWDs on the Hertzsprung–Russell diagram (HRD), which indicates that an extra cooling delay mechanism such as crystallization and elemental sedimentation may exist in UMWDs. Further studies have suggested that some UMWDs should have experienced fairly long cooling delays, implying that they are carbon–oxygen (CO) WDs. However, the formation mechanism of these UMCOWDs is still under debate. In this work, we investigate whether the mergers of massive CO WDs with helium WDs (He WDs) can evolve to UMCOWDs. By employing the stellar evolution code mesa, we construct double WD merger remnants to investigate their final fates. We found that the post-merger evolution of the remnants is similar to R CrB stars. The helium burning of the He shell leads to mass growth of the CO core at a rate of 2.0 × 10−6–$5.0\times {10}^{-6}\, {\rm M}_\odot \, \rm {yr}^{-1}$. The final CO WD mass is influenced by the wind mass-loss rate during the post-merger evolution, and cannot exceed about 1.2M⊙. Remnants with core masses larger than $1.2\, {\rm M}_\odot$ will experience surface carbon ignition, which may finally end their lives as ONe WDs. The current results imply that at least some UMWDs that experience extra-long cooling delays may stem from the merging of CO WDs and He WDs.

Formation of ultra-massive carbon-oxygen white dwarfs from the merger of carbon-oxygen and helium white dwarf pairs

Ultra-massive white dwarfs (UMWDs) with masses larger than 1.05Msun are basically believed to harbour oxygen-neon (ONe) cores. Recently, Gaia data reveals an enhancement of UMWDs on Hertzsprung-Russell diagram (HRD), which indicates that extra cooling delay mechanism such as crystallization and elemental sedimentation may exist in the UMWDs. Further studies suggested that a portion of UMWDs should have experienced pretty long cooling delays, implying that they are carbon-oxygen (CO) WDs. However, the formation mechanism of these UMCOWDs is still under debate. In this work, we investigated whether the merges of massive CO WDs with helium WDs (He WDs) can evolve to UMCOWDs. By employing stellar evolution code MESA, we construct double WD merger remnants to investigate their final fates. We found that the post-merger evolution of the remnants are similar to R CrB stars. The helium burning of the He shell leads to the mass growing of the CO core at a rate from 2.0*10^-6 to 5.0*10^-6 Msun/yr . The final CO WD mass is influenced by the wind-mass-loss rate during the post-merger evolution, and cannot exceed about 1.2Msun. The remnants with core mass larger than 1.2Msun will experience surface carbon ignition, which may finally end their lives as ONe WDs. Current results implies that at least a portion of UMWDs which experience extra long cooling delay may stem from merging of CO WDs and He WDs.

New Blue and Red Variable Stars in NGC 247

Abstract Images recorded with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South are combined with archival images from other facilities to search for variable stars in the southern half of the nearby disk galaxy NGC 247. Fifteen new periodic and nonperiodic variables are identified. These include three Cepheids with periods <25 days, four semiregular variables, one of which shows light variations similar to those of R CrB stars, five variables with intrinsic visible/red brightnesses and colors that are similar to those of luminous blue variables (LBVs), and three fainter blue variables, one of which may be a noneclipsing close binary system. The S Doradus instability strip defines the upper envelope of a distinct sequence of objects on the (i, g−i) color–magnitude diagram (CMD) of NGC 247. The frequency of variability of an amplitude ≥0.1 magnitude in the part of the CMD that contains LBVs over the seven-month period when the GMOS images were recorded is ∼0.2. The light curve of the B[e] supergiant J004702.18–204739.9, which is among the brightest stars in NGC 247, is also examined. Low-amplitude variations on day-to-day timescales are found, coupled with a systematic trend in mean brightness over a six-month time interval.

A new infrared photometric study of hydrogen-deficient carbon stars, R CrB stars and extreme helium stars in our galaxy

We have collected almost all HdC, RCB and EHe stars up to date to investigate their infrared properties, in particular, their radiation mechanisms in the infrared, using the observational data from 2MASS, WISE and IRAS missions. Because HdC stars, RCB stars and EHe stars all belong to the category of extremely hydrogen-deficient stars and they definitely have some evolutionary connections.It is found from this paper that all RCB stars have infrared excesse in the wavelength region from the near infrared to the far infrared covered by 2MASS, WISE and IRAS due to dust while almost all HdC stars and EHe stars have no or little infrared excess.From 2MASS, WISE and IRAS two-color diagrams, it is also found that the majority of these three kinds of stars are around the power law distributions in the near infrared possibly indicative of their infrared radiations from the warm dust in the disk-like envelope. It is also found that in the mid- and far infrared regions some stars are around the power law distribution perhaps due to dust from disk-like envelope while some stars are around the blackbody distribution maybe due to the cold and extended nebulosity nearby.

The First Ultracompact Roche Lobe–Filling Hot Subdwarf Binary

Abstract We report the discovery of the first short-period binary in which a hot subdwarf star (sdOB) filled its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic plane named the Zwicky Transient Facility and exhibits a period of P = 39.3401(1) minutes, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of = 42,400 ± 300 K and a surface gravity of = 5.77 ± 0.05. A high signal-to-noise ratio GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass M sdOB = 0.337 ± 0.015 and a white dwarf accretor with a mass M WD = 0.545 ± 0.020 . Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5–2.8 star lost its envelope when crossing the Hertzsprung gap. To match its current , , , and masses, we estimate a post–common envelope period of ≈ 150 minutes and find that the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of ≈0.1 , merge with its carbon/oxygen white dwarf companion after ≈17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star.

Evolutionary Models for R Coronae Borealis Stars

Abstract We use Modules for Experiments in Stellar Astrophysics (MESA) to construct stellar evolution models that reach a hydrogen-deficient, carbon-rich giant phase like the R Coronae Borealis (R CrB) stars. These models use opacities from OPAL and ÆSOPUS that cover the conditions in the cool, H-deficient, CNO-enhanced envelopes of these stars. We compare models that begin from homogeneous He stars with models constructed to reproduce the remnant structure shortly after the merger of a He and a CO white dwarf (WD). We emphasize that models originating from merger scenarios have a thermal reconfiguration phase that can last up to post-merger, suggesting some galactic objects should be in this phase. We illustrate the important role of mass loss in setting the lifetimes of the R CrB stars. Using asymptotic giant branch-like mass-loss prescriptions, models with CO WD primaries typically leave the R CrB phase with total masses , roughly independent of their total mass immediately post-merger. This implies that the descendants of the R CrB stars may have a relatively narrow range in mass and luminosity as extreme He stars and a relatively narrow range in mass as single WDs.

The OmegaWhite Survey for Short-period Variable Stars. V. Discovery of an Ultracompact Hot Subdwarf Binary with a Compact Companion in a 44-minute Orbit

Abstract We report the discovery of the ultracompact hot subdwarf (sdOB) binary OW J074106.0–294811.0 with an orbital period of minutes, making it the most compact hot subdwarf binary known. Spectroscopic observations using the VLT, Gemini and Keck telescopes revealed a He-sdOB primary with an intermediate helium abundance, = K and = 5.74 ± 0.09. High signal-to-noise ratio light curves show strong ellipsoidal modulation resulting in a derived sdOB mass with a WD companion ( ). The mass ratio was found to be . The derived mass for the He-sdOB is inconsistent with the canonical mass for hot subdwarfs of . To put constraints on the structure and evolutionary history of the sdOB star we compared the derived , , and sdOB mass to evolutionary tracks of helium stars and helium white dwarfs calculated with Modules for Experiments in Stellar Astrophysics (MESA). We find that the best-fitting model is a helium white dwarf with a mass of 0.320 , which left the common envelope ago, which is consistent with the observations. As a helium white dwarf with a massive white dwarf companion, the object will reach contact in 17.6 Myr at an orbital period of 5 minutes. Depending on the spin–orbit synchronization timescale the object will either merge to form an R CrB star or end up as a stably accreting AM CVn-type system with a helium white dwarf donor.

MOST DOUBLE DEGENERATE LOW-MASS WHITE DWARF BINARIES MERGE

ABSTRACT We estimate the merger rate of double degenerate binaries containing extremely low mass (ELM; M ⊙) white dwarfs (WDs) in the Galaxy. Such WDs are detectable for timescales of 0.1–1 Gyr in the ELM Survey; the binaries they reside in have gravitational wave merger times of 0.001–100 Gyr. To explain the observed distribution requires that most ELM WD binary progenitors detach from the common envelope phase with <1 hr orbital periods. We calculate the local space density of ELM WD binaries and estimate a merger rate of 3 × 10−3 yr−1 over the entire disk of the Milky Way; the merger rate in the halo is 10 times smaller. The ELM WD binary merger rate exceeds by a factor of 40 the formation rate of stable mass transfer AM CVn binaries, marginally exceeds the rate of underluminous supernovae, and is identical to the formation rate of R CrB stars. On this basis, we conclude that ELM WD binaries can be the progenitors of all observed AM CVn and possibly underluminous supernovae; however, the majority of He+CO WD binaries go through unstable mass transfer and merge, e.g., into single massive ∼1 M ⊙ WDs.

SAGE-VAR: AN INFRARED SURVEY OF VARIABILITY IN THE MAGELLANIC CLOUDS

We present the first results from the SAGE-Var program, a follow on to the Spitzer legacy program Surveying the Agents of Galaxy Evolution (SAGE; Meixner, et al. 2006). We obtained 4 epochs of photometry at 3.6 & 4.5 microns covering the bar of the Large Magellanic Cloud (LMC) and the central region of the Small Magellanic Cloud (SMC) in order to probe the variability of extremely red sources missed by variability surveys conducted at shorter wavelengths, and to provide additional epochs of observation for known variables. Our 6 total epochs of observations allow us to probe infrared variability on 15 different timescales ranging from ~20 days to ~5 years. Out of a full catalog of 1,717,554 (LMC) and 457,760 (SMC) objects, we find 10 (LMC) and 6 (SMC) large amplitude AGB variables without optically measured variability owing to circumstellar dust obscuration. The catalog also contains multiple observations of known AGB variables, type I and II Cepheids, eclipsing variables, R CrB stars and young stellar objects which will be discussed in following papers. Here we present infrared Period-Luminosity (PL) relations for classical Cepheids in the Magellanic Clouds, as well as improved PL relationships for AGB stars pulsating in the fundamental mode using mean magnitudes constructed from 6 epochs of observations.

New light on Galactic post-asymptotic giant branch stars – I. First distance catalogue

We have commenced a detailed analysis of the known sample of Galactic post-asymptotic giant branch (PAGB) objects compiled in the Toru\'n catalogue of Szczerba et al., and present, for the first time, homogeneously derived distance determinations for the 209 likely and 87 possible catalogued PAGB stars from that compilation. Knowing distances are essential in determining meaningful physical characteristics for these sources and this has been difficult to determine for most objects previously. The distances were determined by modelling their spectral energy distributions (SED) with multiple black-body curves, and integrating under the overall fit to determine the total distance-dependent flux. This method works because the luminosity of these central stars is very nearly constant from the tip of the AGB phase to the beginning of the white-dwarf cooling track. This then enables us to use a standard-candle luminosity to estimate the SED distances. For Galactic thin disk PAGB objects, we use three luminosity bins based on typical observational characteristics, ranging between 3500 and 12000 L_sun. We further adopt a default luminosity of 1700 L_sun for all halo PAGB objects. We have also applied the above technique to a further sample of 69 related nebulae not in the current edition of the Toru\'n catalogue. In a follow-up paper we will estimate distances to the subset of RV Tauri variables using empirical period-luminosity relations, and to the R\,CrB stars, allowing a population comparison of these objects with the other subclasses of PAGB stars for the first time.

Analysis of the spectral energy distributions of four cool R CrB stars near the brightness maximum

The effective temperatures Teff and carbon and nitrogen abundances in the atmospheres of the cool R CrB stars ES Aql, SV Sge, Z UMi, and NSV 11154 have been determined by modeling their spectral energy distributions in the optical and near-infrared. The hydrogen-deficient model atmospheres were computed using the SAM12 code in the classical approximation, taking into account sources of opacity characteristic of the atmospheres of R CrB stars. The influence of the hydrogen deficiency on theoretical stellar spectra is analyzed. The resulting effective-temperature estimates for ES Aql, SV Sge, Z UMi, and NSV 11154 are in the range Teff = 4600–5200 K. The carbon abundances log n(C) in the atmospheres of ES Aql, SV Sge, and Z UMi are 8.9–10.1, corresponding to [C/Fe] values typical of the atmospheres of R CrB stars. The nitrogen abundances are lower than those determined in other studies, and differ considerably from star to star. The mean [N/Fe] value for these three stars is ≈1.5 dex lowthan the mean [N/Fe] for known warm R CrB stars. Abnormally high estimates were obtained for the atmosphere of NSV 11154: log n(C) = 10.8 and log n(N) = 10.0. The approximate log g estimates agree with the conclusion from photometric observations that cool R CrB stars have lower luminosities than hotter R CrB stars.

Can R Coronae Borealis stars form from the merger of two helium white dwarfs?

Abstract Due to orbital decay by gravitational wave radiation, some close binary helium white dwarfs are expected to merge within a Hubble time. The immediate merger products are believed to be He-rich subdwarf O (sdO) stars, essentially helium main-sequence stars. We present new evolution calculations for these post-merger stars beyond the core He-burning phase. The most massive He-rich sdO stars develop a strong He-burning shell and evolve to become He-rich giants. We include nucleosynthesis calculations following the merger of 0.4 M⊙ He white dwarf pairs with metallicities Z= 0.0001, 0.004, 0.008 and 0.02. The surface chemistries of the resulting giants are in partial agreement with the observed abundances of R Coronae Borealis (R CrB) and extreme He stars. Such stars might represent a third, albeit rare, evolution channel for the latter, in addition to the CO+He white dwarf merger and the very late thermal pulse channels proposed previously. We confirm a recent suggestion that Li seen in R CrB stars could form naturally during the hot phase of a merger in the presence of 3He from the donor white dwarf.

Lithium production in the merging of white dwarf stars

The origin of R Coronae Borealis stars has been elusive for over 200 years. Currently, two theories for their formation have been presented. These are the final flash scenario, in which a dying asymptotic giant branch star throws off its atmosphere to reveal the hydrogen poor, heavily processed material underneath, and the double degenerate scenario, in which two white dwarfs merge to produce a new star with renewed vigour. Some theories predict that the temperatures reached during the latter scenario would destroy any lithium originally present in the white dwarfs. The observed lithium content of some R Coronae Borealis stars, therefore, is often interpreted as an indication that the final flash scenario best describes their formation. In this paper, we show that lithium production can, indeed, occur in the merging of a helium white dwarf with a carbon-oxygen white dwarf if their chemical composition, particularly that of 3He, is fully considered. The production mechanism is described in detail, and the sensitivity of lithium production to the merger environment is investigated. Nucleosynthesis post-processing calculations of smoothed-particle hydrodynamics (SPH) tracer particles are performed to show that any lithium produced in these environments will be concentrated towards the cloud of material surrounding the R CrB star. Measurements of the lithium content of these clouds would, therefore, provide a valuable insight into the formation mechanism of R CrB stars.

Light Echoes

AbstractThe first “light echo”—scattered light from a stellar outburst arriving at the Earth months or years after the direct light from the event—was detected more than 100 years ago, around Nova Persei 1901. Renewed interest in light echoes has come from the spectacular echo around V838 Monocerotis, and from discoveries of light echoes from historical and prehistorical supernovæ in the Milky Way and Large Magellanic Cloud as well as from the 19th-century Great Eruption of η Carinae. A related technique is reverberation mapping of active galactic nuclei. This report of a workshop on Light Echoes gives an introduction to light echoes, and summarizes presentations on discoveries of light echoes from historical and prehistorical events, light and shadow echoes around R CrB stars, and reverberation mapping.

The formation of Polycyclic Aromatic Hydrocarbons in evolved circumstellar environments

The formation of Polycyclic Aromatic Hydrocarbons in the circumstellar outflows of evolved stars is reviewed, with an emphasis on carbon stars on the Asymptotic Giant Branch. Evidence for PAHs present in their winds is provided by meteoritic studies and recent observations of the Unidentified Infrared bands. We detail the chemical processes leading to the closure of the first aromatic ring as well as the growth mechanisms leading to amorphous carbon grains. Existing studies on PAH formation in evolved stellar envelopes are reviewed and new results for the modelling of the inner wind of the archetype carbon star IRC+10216 are presented. Benzene, C6H6, forms close to the star, as well as water, H2O, as a result of non-equilibrium chemistry induced by the periodic passage of shocks. The growth process of aromatic rings may thus resemble that active in sooting flames due to the presence of radicals like hydroxyl, OH. Finally, we discuss possible formation processes for PAHs and aromatic compounds in the hydrogen-rich R CrB star, V854 Cen, and their implication for the carriers of the Red Emission and the Diffuse Interstellar Bands.

Binaries discovered by the SPY project

<i>Aims. <i/>The ESO SN Ia Progenitor Survey (SPY) aims at finding merging double degenerate binaries as candidates for supernova type Ia (SN Ia) explosions. A white dwarf merger has also been suggested to explain the formation of rare types of stars like R CrB, extreme helium or He sdO stars. Here we present the hot subdwarf B binary GD 687, which will merge in less than a Hubble time.<i>Methods. <i/>The orbital parameters of the close binary have been determined from time resolved spectroscopy. Since GD 687 is a single-lined binary, the spectra contain only information about the subdwarf primary and its orbit. From high resolution spectra the projected rotational velocity was derived. Assuming orbital synchronisation, the inclination of the system and the mass of the unseen companion were constrained.<i>Results. <i/>The derived inclination is <i>i<i/> = °. The mass <i>M<i/><sub>2<sub/> = indicates that the companion must be a white dwarf, most likely of C/O composition. This is only the fourth case that an sdB companion has been proven to be a white dwarf unambiguously. Its mass is somewhat larger than the average white dwarf mass, but may be as high as 0.93 in which case the total mass of the system comes close to the Chandrasekhar limit.<i>Conclusions. <i/> GD 687 will evolve into a double degenerate system and merge to form a rare supermassive white dwarf with a mass in excess of solar. A death in a sub-Chandrasekhar supernova is also conceivable.

Stellar winds and mass loss from extreme helium stars

Extreme helium stars are very rare low-mass supergiants in a late stage of evolution. They are probably contracting to become white dwarfs following a violent phase of evolution which caused them to become hydrogen-deficient giants, possibly R CrB stars. Using the latest generation of models for spherically expanding stellar atmospheres, we set out to measure mass-loss rates for a representative fraction of these star s. We have used high-resolution ultraviolet and optical spectra, and ultraviolet, optical and near-infrared photometry from a variety of archives. Overall atmospheric parameters have mostly been taken from previous analyses and checked for consistency. Mass-loss rates were measured by fitting the PCygni and asymmetric profiles of C, N and Si ultraviolet reson ance lines and lie in the range 10 −10 10 −7 M⊙yr −1 . These rates follow a Castor-type ( u M / L 1.5 ) relation marking a lower limit for the mass loss from hot stars of all kinds. The mass-loss rates of the studied stars also show a strong correlation with their proximity to the Eddington limit. There is no firm evidence for variability in the stellar wind, although p hotospheric pulsations have been reported in many cases.