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While the first binary post-AGB stars were serendipitously discovered, the distinct characteristics of their Spectral Energy Distribution (SED) allowed us to launch a more systematic search for binaries. We selected post-AGB objects which show a broad dust excess often starting already at H or K, pointing to the presence of a gravitationally bound dusty disc in the system. We started a very extensive multi-wavelength study of those systems and here we report on our radial velocity and photometric monitoring results for six stars of early F type, which are pulsators of small amplitude. To determine the radial velocity of low signal-to-noise time-series, we constructed dedicated auto-correlation masks. The radial velocity variations were subjected to detailed analysis to differentiate between pulsational variability and variability due to orbital motion. Finally orbital minimalisation was performed to constrain the orbital elements. All of the six objects are binaries, with orbital periods ranging from 120 to 1800 days. Five systems have non-circular orbits. The mass functions range from 0.004 to 0.57 solar mass and the companions are likely unevolved objects of (very) low initial mass. We argue that these binaries must have been subject to severe binary interaction when the primary was a cool supergiant. Although the origin of the circumstellar disc is not well understood, the disc is generally believed to be formed during this strong interaction phase. The eccentric orbits of these highly evolved objects remain poorly understood. With the measured orbits and mass functions we conclude that the circumbinary discs seem to have a major impact on the evolution of a significant fraction of binary systems., 13 pages, 15 figures, accepted for Astronomy and Astrophysics

We discuss ISO spectroscopy of oxygen-rich dust shells surrounding evolved stars. The dust that condenses in the outflows of stars on the Asymptotic Giant Branch consists mainly of amorphous silicates and simple oxides. For high mass loss rates, crystalline silicates begin to appear at modest abundance. These crystalline silicates are cold and Fe-poor. ISO spectroscopy for the first time allows quantitative mineralogy of oxygen-rich circumstellar dust. Crystalline silicates are found at high abundance in sources with peculiar (disk) geometry, such as long-lived circum-binary disks. Some C-rich post-AGB stars, notably the Red Rectangle and several nebulae surrounding [WC] central stars of Planetary Nebulae, also show crystalline silicates. We speculate on the origin and evolution of the crystalline dust component in evolved stars.

We present a list of post-AGB stars showing molecular line absorption and emission in the optical spectrum. Two objects show CH+, one in emission and one in absorption, and 10 stars show C-2 and CN in absorption. The Doppler velocities of the C-2 lines and the rotational temperatures indicate that the line forming region is the AGB remnant. An analysis of the post-AGB stars of which CO millimeter data is available suggests that the C-2 expansion velocity is of the same order as the CO expansion velocity. HD 56126 has been studied in detail and we find a mass-loss rate of M=2.8 x 10(-4) M.yr(-1), f(C2) = 2.4 x 10(-8) and f(CN) = 1.3 x 10(-8). The mass-loss derived from C-2 is significantly; larger than M = 1.2 x 10(-5) M.yr(-1) derived from CO. We find that all objects with the 21 mu m feature in emission show C-2 and CN absorption, but not all objects with C-2 and CN detections show a 21 mu m feature.

We present a list of post-AGB stars showing molecular line absorption and emission in the optical spectrum. Two objects show CH+, one in emission and one in absorption, and 10 stars show C-2 and CN in absorption. The Doppler velocities of the C-2 lines and the rotational temperatures indicate that the line forming region is the AGB remnant. An analysis of the post-AGB stars of which CO millimeter data is available suggests that the C-2 expansion velocity is of the same order as the CO expansion velocity. HD 56126 has been studied in detail and we find a mass-loss rate of M=2.8 x 10(-4) M.yr(-1), f(C2) = 2.4 x 10(-8) and f(CN) = 1.3 x 10(-8). The mass-loss derived from C-2 is significantly; larger than M = 1.2 x 10(-5) M.yr(-1) derived from CO. We find that all objects with the 21 mu m feature in emission show C-2 and CN absorption, but not all objects with C-2 and CN detections show a 21 mu m feature.

Although the presence of circumbinary disks around binary post-AGB stars is now well established, their actual structure, formation, stability and evolution remain largely unknown. We therefore used the MIDI interferometer to observe the disks around a sample of nine post-AGB binaries. These measurements show the very compact nature of the circumstellar environment and confirm the disk interpretation of their spectral energy distribution. In addition, a large diversity is observed in the radial distribution of the crystallinity over the N-band. While some objects show a homogeneous distribution of the crystallinity, for some objects the crystallinity is confined to the innermost regions. Whether this is a result of the formation process or due to annealing during the long storage time in the disk is not clear. 12 pages, 10 figures, accepted for publication in A & A ispartof: Baltic Astronomy vol:16 issue:1 pages:145-147 ispartof: location:POLAND, Wierzba status: published

We present the Spectral Energy Distribution of HR 4049 based on literature data and new continuum measurements at 850 mum. The SED shows variable absorption in the UV, and a large IR excess, both caused by circumstellar dust. The shape of the IR excess from 1 mum all the way down to 850 mum can be nearly perfectly fitted with a single blackbody function at T approximate to1150 K or alternatively with a sum of blackbodies in a narrow temperature range. The energy emitted in this IR continuum radiation is about one-third of the stellar luminosity. We show that this blackbody radiation must be due to the presence of a circumbinary disk with a large height. This disk must also be gas-rich, in agreement with the observations of molecular bands in the ISO-SWS spectrum. We present two possible scenario's for explaining the shape and the intensity of the IR excess. The first scenario involves large grains (a greater than or equal to 1 mm) that each radiate like a blackbody. The second scenario argues that the blackbody radiation is due to a very optically thick circumbinary disk. We investigate if such a disk would indeed produce blackbody radiation by presenting results from radiative transfer calculations. We further quantify the properties of such a disk and its stability in the framework of (hydro)dynamics, grain settling, radiation pressure and grain drift. The virtues and shortcomings of both models for the origin of the IR blackbody are discussed by contrasting them with other observations and assessing them in the framework of (binary) (post-)AGB evolution.

This is the first paper in a series of three where we present the first comprehensive inventory of solid state emission bands observed in a sample of 17 oxygen-rich circumstellar dust shells surrounding evolved stars. The data were taken with the Short and Long Wavelength Spectrographs on board of the Infrared Space Observatory (ISO) and cover the 2.4 to 195 micron wavelength range. The spectra show the presence of broad 10 and 18 micron bands that can be attributed to amorphous silicates. In addition, at least 49 narrow bands are found whose position and width indicate they can be attributed to crystalline silicates. Almost all of these bands were not known before ISO. We have measured the peak positions, widths and strengths of the individual, continuum subtracted bands. Based on these measurements, we were able to order the spectra in sequence of decreasing crystalline silicate band strength. We found that the strength of the emission bands correlates with the geometry of the circumstellar shell, as derived from direct imaging or inferred from the shape of the spectral energy distribution. This naturally divides the sample into objects that show a disk-like geometry (strong crystalline silicate bands), and objects whose dust shell is characteristic of an outflow (weak crystalline silicate bands). All stars with the 33.6 micron forsterite band stronger than 20 percent over continuum are disk sources. We define spectral regions (called complexes) where a concentration of emission bands is evident, at 10, 18, 23, 28, 33, 40 and 60 micron. We derive average shapes for these complexes and compare these to the individual band shapes of the programme stars., 41 pages, 20 figures, accepted by A&A. Tables 4 to 20 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/

We discuss ISO spectroscopy of oxygen-rich dust shells surrounding evolved stars. The dust that condenses in the outflows of stars on the Asymptotic Giant Branch consists mainly of amorphous silicates and simple oxides. For high mass loss rates, crystalline silicates begin to appear at modest abundance. These crystalline silicates are cold and Fe-poor. ISO spectroscopy for the first time allows quantitative mineralogy of oxygen-rich circumstellar dust. Crystalline silicates are found at high abundance in sources with peculiar (disk) geometry, such as long-lived circum-binary disks. Some C-rich post-AGB stars, notably the Red Rectangle and several nebulae surrounding [WC] central stars of Planetary Nebulae, also show crystalline silicates. We speculate on the origin and evolution of the crystalline dust component in evolved stars.

We discuss ISO spectroscopy of oxygen-rich dust shells surrounding evolved stars. The dust that condenses in the outflows of stars on the Asymptotic Giant Branch consists mainly of amorphous silicates and simple oxides. For high mass loss rates, crystalline silicates begin to appear at modest abundance. These crystalline silicates are cold and Fe-poor. ISO spectroscopy for the first time allows quantitative mineralogy of oxygen-rich circumstellar dust. Crystalline silicates are found at high abundance in sources with peculiar (disk) geometry, such as long-lived circum-binary disks. Some C-rich post-AGB stars, notably the Red Rectangle and several nebulae surrounding [WC] central stars of Planetary Nebulae, also show crystalline silicates. We speculate on the origin and evolution of the crystalline dust component in evolved stars.

We present a list of post-AGB stars showing molecular line absorption and emission in the optical spectrum. Two objects show CH+, one in emission and one in absorption, and 10 stars show C-2 and CN in absorption. The Doppler velocities of the C-2 lines and the rotational temperatures indicate that the line forming region is the AGB remnant. An analysis of the post-AGB stars of which CO millimeter data is available suggests that the C-2 expansion velocity is of the same order as the CO expansion velocity. HD 56126 has been studied in detail and we find a mass-loss rate of M=2.8 x 10(-4) M.yr(-1), f(C2) = 2.4 x 10(-8) and f(CN) = 1.3 x 10(-8). The mass-loss derived from C-2 is significantly; larger than M = 1.2 x 10(-5) M.yr(-1) derived from CO. We find that all objects with the 21 mu m feature in emission show C-2 and CN absorption, but not all objects with C-2 and CN detections show a 21 mu m feature.