The long period (P = 27.1 yr) eclipsing binary є Aurigae (F0 Ia + disk?) is truly an exotic star. It has a deep eclipse that lasts for nearly two years. This eclipse arises as a huge, cool, flattened disk transits the F-supergiant star. Modeling of the eclipse observations shows that the disk has a radius as large as ∼ 9 AU. Infrared observations indicate that the disk is cool with temperatures between 450 − 1000 K. Yet there is evidence of significant FUV emission also originating from the disk.

At present, our knowledge of the mass and luminosity of the binary is still too uncertain to distinguish between two competing models of the system. The high mass model assumes that the F0 supergiant is a normal Pop. I star with a mass appropriate for its spectral type of M ∽ 15 M⊙. It is accompanied by a flattened disk companion with a slightly smaller mass. In this model the disk object is a young proto-stellar or protoplanetary disk. In the low mass model, the F0I star is assumed to be a bloated, old, solar mass post-AGB star. In this case the secondary object is an accretion disk with a mass of 4-5 M⊙. This disk is a remnant of postmain sequence mass transfer that occurred within the last few thousand years. In both models there are still problems explaining the object (or objects) at the center of the disk. Candidates include a pre-main sequence object, a black hole, or a close binary.

In this paper we review the properties of ϵ Aurigae and discuss the advances in our understanding of this enigmatic star from observations made since its last eclipse in 1982-1984. With new technologies and advanced instrumentation it is possible that the physical properties of this puzzling binary star will be found during the next decade. Once found, then ϵ Aurigae and its eclipses can be used as a laboratory for exploring (and testing) current astrophysical concepts and theories that include rapid stages of stellar evolution, binary star evolution, and the structure and dynamics of large disks.

V505 Mon is an early type eclipsing binary (usually classified as B5 Ib), in which only spectral lines of the primary component are visible. The radial velocity curve provides f(m) = 4.6 M⊙, suggesting that the secondary, invisible component is more massive. We have found evidence that this star is hidden from view by the disk detected during the primary eclipse. The structure of the disk can be characterized by a central ionization zone surrounded by the neutral outer parts. The profiles of the CII 1335/6 doublet lines were used to derive the rotational velocity of the disk as well as the velocity of the stream from the primary component to the disk. The masses of the components, deduced from the fractional size of the disk, are M1 = 1 M⊙ and M2 = 6.1 M⊙. Due to the low mass of the primary component its supergiant nature can be ruled out. Additional matter of a cloudy nature is present in the system and both components are embedded in an extended atmosphere. V505 Mon seems to be a prototype of the whole group of similar early-type binaries, in which the more massive component is hidden by a circumstellar medium, most likely a disk. Probable members are V742 Cas, V1362 Cyg, V2174 Cyg, V447 Sct and FY Vel with orbital periods between 33 and 225 days. Emission in Ha is always present, but there are no emissions in IUE spectra, so these binaries are distinct from the W Ser type. The evolutionary state of the members of this group is not yet understood.

Current information on the nature of the HR 2142 system is presented. The circumstellar material (CSM) including the mass flow toward and away from the mass-gaining B star has been studied from ∼ 100 IUESWP HIRES images obtained from 1979–95 and ground-based CCD data from KPNO acquired between 1985–2001. The strength and velocity behavior of the infall components to the UV Si ɪɪ lines seen during the primary shell phase (PSP) from Φ ∼ 0.70−0.98 resembles that observed in conventional Algol systems, but the inferred mass infall rate is too small to account for the massive Hα-emitting disk about the B primary. The cause for the mass outflow observed during the secondary shell phase (SSP) remains unknown, but a localized plasma that has been identified from phase-dependent variations in the emission component to Heɪ λ6678 may be part of the SSP phenomenon. Although the secondary remains undetected in the optical/IR spectral regions, UV data currently do not yield strong support for the presence of an O subdwarf as in the Φ Per system. HR 2142 may represent and earlier evolutionary stage of Φ Per and a possibly similar Be binary 59 Cyg.

AS Camelopardalis is notorious for its apparent disregard for the theory of apsidal motion. The orbit of this 3.43-day double-lined eclipsing binary (B8V + B9.5V) rotates approximately 15° per century, which is only 0.3 to 0.4 as fast as predicted (classical + relativistic). Our dynamical program explores, as have others, the suggestion that a third star perturbs the orbit of the eclipsing pair and slows its apsidal motion, dω/dt. On the observational side, we unify the process of extracting apsidal motion and light-time effect parameters from eclipse minima by directly including a third body light-time effect along with the usual ephemeris parameters. By numerical experiment we identify third body parameters consistent with the eclipse timings and with the dynamics of third body perturbations. Results include the minimum third body mass required to produce the given retrograde apsidal rate. Finally, we use a general binary star light curve program that includes a light-time effect to solve for dω/dt and ephemeris parameters together with other quantities, combining 21 years of radial velocities and light curves within one coherent analysis. The program has an improved stellar atmosphere routine based on Legendre polynomials that have been fitted to Kurucz atmosphere models. By analyzing whole light curves, the program has access to more information than only times of minima. Results agree well with those from eclipse timings over 100 years and attain smaller standard errors despite utilizing only one-fifth of the traditional method’s baseline in time.

A fluorescent light curve model (ionization-bounded wind and chromosphere with scale height) that previously found most fluorescent variation in symbiotic nova V1329 Cygni to be chromospheric, is applied to AG Draconis, where the strong wind of a well-detached red giant is illuminated by ultraviolet radiation from an accreting white dwarf. Bandpass dependence of light curve amplitude is even more striking than in V1329 Cyg and eliminates thermal re-radiation from the red star photosphere as a principal variation mechanism, since thermal re-radiation cannot change by large factors from U to B to V. Spectra show strong continuum and line emission, so both light curves and spectra show the importance of fluorescence. A small secular decay was included by fitting in time rather than phase, with dTwd/dt as a parameter. There is a 3σ indication of a large negative dP/dt. Chromospheric fluorescence accounts for the entire U variation, while B and V variations are too small for wind and chromosphere to be distinguished. We confirm a previously identified 355-day radial velocity period that is consistent with red star pulsation.

The red dwarf + white dwarf eclipsing binary V471 Tau shows a variable Hα feature that varies from absorption during eclipse to maximum emission during white dwarf transit. In 1998 we obtained simultaneous BVRI photometry and Hα spectroscopy, with thorough phase coverage of the 12.5 hour orbital period. A binary star model was used with our light curve, radial velocity, and Hα data to refine stellar and orbital parameters. Combined absorption-emission profiles were generated by the model and fit to the observations, yielding a red star radius of 0.94R⊙. Orbital inclination 78° is required with this size and other known parameters. The model includes three spots 1,000 K cooler than the surrounding photosphere. The variable Hα profile was modeled as a chromospheric fluorescing region (essentially on the surface of the red star) centered at the substellar point. Additional emission seen outside our modeled profiles may be large co-rotating prominences that complicate the picture.

FG Sagittae has evolved from a hot central star of a planetary nebula to an K-type supergiant within approximately the last 100 years. The generally accepted interpretation for this redward evolution is that of a late thermal pulse during the planetary nebula stage, wherein helium burning at the surface of the electron-degenerate carbon-oxygen core is reignited. As the star expands in response to the energy released by helium burning, envelope convection digs deeper and deeper until nuclearly processed material may get dredged-up to the stellar surface. Analysing the spectra as FG Sge is evolving would then give unique information about the temporal development of mixing processes occurring inside the star that are otherwise impossible to obtain. The existing abundance analyses do not give, however, a consistent picture. Especially the question about FG Sge’s hydrogen abundance is still unsettled. We present a critical assessment of all the existing data, trying to find a self-consistent picture of the evolution of FG Sge, based on the latest evolutionary models.

The cloud of ejecta resulting from a final helium shell flash in V605 Aql has been imaged in both optical emission lines and infrared continuum. The obscuring circumstellar dust cloud, whose effects were first seen in 1923, is shown to be bipolar, suggesting dust formation takes place in a disk surrounding this star. The ongoing stellar mass loss in V605 Aql is part of the final flash episode, which has been ongoing since 1919. The pre-final flash planetary nebula, A58, is nearly circular. This implies that significant changes in the mass loss process occur between the AGB and final flash stages.

We have completed a grid of stellar evolution calculations to study the behavior of the born again phenomenon. All our evolutionary sequences begin with a uniform composition 1 M⊙ star on the pre-main sequence Hayashi phase and end on the white dwarf cooling track. We find a very late thermal pulse occurs in 10 – 15 percent of cases. Our models supply an answer to the question of why the born again stars V4334 Sgr (Sakurai’s Object) and V605 Aql have a significantly shorter evolutionary time scale than the otherwise similar born again star FG Sge. Models with low convective mixing efficiency, η = 10–4, first evolve quickly to the AGB, return to the blue, and then evolve more slowly back to the AGB for a second time before finally returning to the white dwarf cooling track. The difference in evolution time scales can then be explained by proposing that Sakurai’s Object is evolving to the AGB for the first time but FG Sge has been observed during its second return to the AGB.

H 1504+65 is an extremely hot hydrogen-deficient white dwarf with an effective temperature close to 200 000 K. We present new FUV and soft X-ray spectra obtained with FUSE and Chandra, which confirm that H1504+65 has an atmosphere primarily composed of carbon and oxygen. The Chandra spectra show a wealth of absorption lines from highly ionized oxygen, neon and magnesium and suggest relatively high Ne and Mg abundances. This corroborates an earlier suggestion that H1504+65 represents a naked C/O stellar core or even the C/O envelope of an O/Ne/Mg white dwarf.

Accurate simulation of optical wavelength CCD spectra is possible by adopting a black-body representation for the radiation characteristics of the β Lyrae accretion disk rim. Application of this model to IUE spectra shows that the anomalous system light curves, in the satellite UV, arise from a source of radiation, not included in the current model, which is visible during the phase 0.0 eclipse.

The inner region of the accretion disk around a magnetized star is subjected to magnetic torques that induce warping and precession of the disk. These torques arise from interactions between the stellar field and the induced electric currents in the disk. These novel magnetic effects give rise to some “exotic” stellar variabilities, and may play an important role in explaining a number of puzzling behaviors related to disk accretion onto magnetic stars, such as mHz QPOs in X-ray pulsars, long-term periodicities of X-ray binaries (including precession of jets), low-Frequency (10-50 Hz) QPO’s in low-mass X-ray binaries, and photometric variabilities of T Tauri stars.

γ Cas (HD 5394) is an important representative of Be stars, hot B stars exhibiting Balmer and other emission lines in their spectra at certain epochs. Its emission spectrum was observed as early as in 1863 and has undergone spectacular changes since then. γ Cas is also extremely interesting because it qualifies for several phenomenologically defined groups: it is a light and colour variable, long-term radial-velocity variable, X-ray source, IRAS source, rapid line-profile variable, a single-line spectroscopic binary and a central star of a reflection nebula.

A critical compilation of the wealth of observational data on the star, with emphasis on its time variability on several time scales, as well as an estimate of the most probable values of all basic physical properties of the object is presented. It is pointed out that in spite of a large quantity of data, systematic and well-calibrated observations are still rather rare. The present understanding of the object is put into perspective of the more general – and as yet unsolved – problem of the formation of Be envelopes.

V471 Tauri is a short-period eclipsing binary, and a member of the Hyades. It is composed of a hot DA white dwarf (WD) and a cool main-sequence dK2 companion. HST radial velocities of the WD, in combination with the ground-based spectroscopic orbit of the K star, yield dynamical masses of MWD = 0.84 and MdK = 0.93 M⊙. During the UV observations we serendipitously detected coronal mass ejections from the K star, passing in front of the WD and appearing as sudden, transient metallic absorption. Eclipse timings show that the active dK star is 18% larger than a main-sequence star of the same mass, an apparent consequence of its extensive starspot coverage. The high Teff and high mass of the WD are paradoxical: the WD is the most massive in the Hyades, but also the youngest. A plausible scenario is that the progenitor system was a triple, with a close inner pair that merged after several × 108 yr to produce a single blue straggler. When this star evolved to the AGB phase, it underwent a common-envelope interaction with a distant dK companion, which spiraled down to its present separation and ejected the envelope. The common-envelope efficiency parameter, αCE, was of order 0.3–1.0, in good agreement with recent hydrodynamical simulations.

SuWt 2 is a southern-hemisphere planetary nebula (PN) with an unusual ring-shaped morphology. The central star is an eclipsing binary with a period of 4.9 days. Surprisingly, the binary is composed of two main-sequence A-type stars with similar masses of ~ 2.5 M⊙. We discuss scenarios involving a third companion which ejected and ionizes the PN.

WeBo 1 is a northern PN with a ring morphology remarkably similar to that of SuWt 2. Although we hoped that its central star would shed light on the nature of SuWt 2, it has proven instead to be a late-type barium star!

We study the slow merger of two massive stars inside a common envelope. The initial close binary system consists of a massive red supergiant and a main-sequence companion of a few solar masses. The merger product is a massive supergiant with an interior structure (core mass and composition profile) which is significantly different from that of a single supergiant that has evolved in isolation. Using a parameterized approach for the stream–core interaction, we modelled the merger phase and have identified three qualitatively different merger types: quiet, moderate and explosive mergers, where the differences are caused by the different response of the He burning shell. In the last two scenarios, the post-merger He abundance in the envelope is found to be substantially increased, but significant s-processing is mainly expected in the case of an explosive merger scenario. The subsequent evolution of the merger product up to the supernova stage is also discussed.

Irradiation of the secondaries in close binary systems affects their appearance and can drastically change their internal structure and hence long-term evolution. In this paper we review the main effects of external irradiation in close binaries with compact primaries, such as the distortion of the shape of the secondary and the conditions for the occurrence of outer critical configurations, and apply these to normal X-ray binaries, including systems such as HZ Her/Her X-l, Vela X-l and black-hole binaries during outburst (e.g. Nova Sco). Particular emphasis will be given to the role of circulation driven by the external heating and the radiative surface stress. We have developed a three-dimensional fluid dynamics code for modelling these effects. We present initial results of self-consistent calculations for the circulation in irradiated systems and show how the inclusion of these effects is vital to the understanding and interpretation of any system where external irradiation of the secondary is significant.

CW Leo is the nearest and brightest example of a highly evolved carbon Mira variable red giant, and is surrounded by an extensive dusty, molecular circumstellar shell produced by mass loss. Since the shell heavily obscures the star, most of the information about CW Leo comes from observations of its shell, whose radial properties encode a history of the star as well as of the interaction between the outflowing gas and the interstellar radiation field. The shell of CW Leo contains a rich variety of molecules, from salts to cyanide, whose isotopic composition shows the effect of nuclear processing in the stellar interior and give information about the formation and composition of the circumstellar dust. This nearby star allows us a detailed look at one of the most eventful and interesting phases in a star’s evolution.

A model of maser emission spots as icy planets orbiting young massive stars is proposed. It is argued that the interstellar gas phase abundance of methanol, H2O and OH is much too low to produce maser emission from small gas clumps. The interstellar grains, on the other hand, consisting mainly of water ice and several per cent of methanol, can produce a favorable environment for the maser emission when evaporated. Big comets or planets covered by the water/methanol ice orbiting young massive stars can be the sites of the maser emission. The ice evaporates under stellar radiation, and water and methanol molecules emit corresponding maser lines, while OH molecules are produced from dissociation of water by the stellar radiation. VLBI observations of masers in W 3(OH), W 75N and V645 Cyg show that proposed planets are found in disks, and are moving on Keplerian orbits around O, B-stars. In the case of the O-star in W 3(OH) the planets are located beyond the compact HII region of the star. Extended envelopes of the icy planets provide molecular abundance, gas temperature and density adequate for the maser emission.

Radio images of maser spots in the infrared source RAFGL 2789, connected with the young stellar object V645 Cyg, have been obtained as a result of radio interferometric observations of the H2O maser at 22 GHz and the methanol maser at 6.7 GHz, with the VLBI arrays VLBA and EVN. It is shown that the position of the masers coincides with the optical object within 02. The maser spots are located along the line North-South, and their position and radial velocity can be described by a model of a Keplerian disk with a maximum radius of 40 AU for the H2O maser and 800 AU for the methanol maser. The H2O and methanol maser spots have not been resolved, and lower limits of the brightness temperature are 2 × 1013 K and 1.4 × 109 k respectively. A model of the maser is suggested in which the maser emission is generated in extended water and methanol envelopes of icy planets orbiting the young star.

We performed full binary evolution calculations and carried out binary population synthesis studies in order to investigate the formation of subdwarf B (sdB) stars via the channels of stable Roche lobe overflow (RLOF), common envelope ejection and helium white dwarf mergers. Our model is successful in the explanation of observational properties of sdB stars, e.g. we explained the orbital period – minimum companion mass (log P – Mcomp) diagram, the effective temperature – surface gravity (Teff – log g) diagram, the orbital period distribution, the log(gθ4) (θ = 5040/Teff) distribution, the mass function distribution, the binary fraction of sdB stars, the fraction of sdB binaries with WD companions, the birth rates, the space number densities, etc. We conclude that (a) the first RLOF needs to be more stabilized than commonly assumed, (b) the first stable RLOF is not conservative, (c) common envelope ejection is very efficient.