As part of the Extragalactic Distance Scale Key Project, the Hubble Space Telescope has been used to identify Cepheids in M100, M101 and NGC925, and to measure distances derived from the Cepheid PL relation. For M100, the distance of 17.1 ± 1.8 Mpc has been used to infer a preliminary value for H0 of ~ 80 km/s/Mpc, which brings the age of the Universe derived from the standard model of the Big Bang into conflict with the ages of the oldest stars.

We present Fourier analysis at 490 nm for 87 Cepheid light curves in the bar of the Large Magellanic Cloud. The photometry has been obtained through the EROS project whose main purpose is the search for baryonic dark matter in the Galactic halo. The very high quality of the photometry and the good phase coverage enable us to obtain accurate Fourier decomposition of the light curves. As a consequence we confirm that the so-called s-Cepheids are overtone pulsators and represent about 30 % of our sample, whereas the Classical Cepheids are fundamental pulsators. We have identified a number of features in plots of amplitude ratios and phase differences versus period. These features are usually related to resonances between different pulsation modes. If we accept that Fourier decomposition techniques applied to light curves yield positions for the centres of resonances, then it seems that resonances occur in LMC Cepheids at the same period as in Galactic Cepheids for fundamental pulsators, whereas they appear to be at shorter periods for overtone pulsators.

The drive for high-angular resolution from the ground has led to the development of optical and near infrared interferometric telescopes analogous to radio synthesis arrays. These now provide a diffraction-limited imaging capability well suited to studies of nearby cool giants and supergiants. In this review I summarise the current status of these instruments, their scientific potential for studies of cool pulsating variables (LPVs), and present some recent results that highlight the promise that such methods hold.

The Sydney University Stellar Interferometer (SUSI) is the first instrument with sufficient resolving power to measure the angular sizes of a large number of stars over the full range of spectral classes. It can also resolve the orbits of spectroscopic binaries, allowing masses and distances to be determined directly. One of the major programs envisaged for the instrument is a study of pulsating stars, in particular Cepheids.

We perform time series analysis on irregularly spaced data with large gaps. Our algorithm is based on multidimensional nonlinear optimisation. Particular emphasis is given to the selection and deletion of frequencies. The final result is checked by various statistical tests.

An asymptotic representation of low-frequency, linear, isentropic g-modes of a star is developed without the usual neglect of the Eulerian perturbation of the gravitational potential. Our asymptotic representation is based on the use of asymptotic expansions adequate for solutions of singular perturbation problems (see, e.g., Kevorkian & Cole 1981).

Linear, isentropic oscillation modes with frequency different from zero are governed by a fourth-order system of linear, homogeneous differential equations in the radial parts of the radial displacement ξ(r) and the divergence α(r). These equations take the form

The symbols have their usual meaning. N2 is the square of the frequency of Brunt-Väisälä. The functions K1 (r), K2 (r), K3 (r), K4 (r), depend on the equilibrium model, e.g.,

We introduce the small expansion parameter

and assume, for the sake of simplification, N2 to be positive everywhere in the star so that the star is everywhere convectively stable.

The strength of nonlinear interactions of oscillation modes of stars is determined by the amplitudes as well as by the eigenfunctions of the oscillation modes. The intrinsic couplings of modes through their eigenfunctions can be described by coupling coefficients. Here, we concentrate on quadratic and cubic coupling coefficients that describe the nonlinear coupling of modes with itself and are called self-coupling coefficients.

We considered radial and nonradial oscillation modes of polytropic models with degrees of central condensation that correspond to central condensations of main sequence stars to highly condensed evolved stars. We study the influence of the radial order and the degree of the oscillation mode on the self- coupling coefficients.

Various observational data and model calculations are used to identify four pulsation modes detected in this star and to infer its properties. It is concluded that the star’s interior must rotate faster than the outer layers.

We discuss the sensitivity of the theoretical B star instability domains to the heavy element abundance Z, the adopted metal mixture, the assumed overshooting from stellar convective cores and the choice of the opacity data.

In April 1994 we started a program of searching for variable B-type stars in young open clusters. The equipment consisted of a 60-cm reflecting telescope and a CCD camera with a Johnson’s B filter. Up to date we obtained nearly 1500 CCD frames of the central regions of four clusters: IC4996, NGC7235, NGC7510, and NGC654. In the range from B 0 to B 5, the photometric accuracy we achieve allows detection of short-period variability with amplitudes of about 5 mmag or greater. In this note we present the short-period variable stars we found in NGC 7235 and NGC 7510.

During the past five years, we monitored the Hα line of β Cep. The strength of the Hα emission, first discovered in 1990, is linearly declining with time. Several options are explored, but a satisfactory explanation for the recurring phases of Hα emission is not yet found.

Recently Aerts et al. (1994, Paper I) discovered multi-periodicity in the variations of optical lines of β Cephei. They identified the frequency of the radial pulsation (f1 = 5.25cycles/day), and additionally found 2 frequencies (f2 = 5.38 c/d, f3 = 4.92 c/d) close to the main frequency. The amplitude of these variations was found to be much smaller than that of the radial pulsation. Aerts et al. (1994) attributed these small variations at the newly found frequencies to non-radial pulsation modes of the star, but labelled the mode-identifications as uncertain.

The northern-hemisphere double cluster h and χ Persei is one of the richest young open clusters. At least half of the brightest stars in the cluster appear to be variables and most of them axe Be stars. Till now there was no evidence for the presence of β Cephei stars in the cluster. It is remarkable that the similar age southern cluster NGC 3293 and other clusters belonging to the same age group are documented as clusters in which β Cephei stars have been observed. In this work we report finding one suspected β Cephei-like object, basing on our own observations.

Two large sinusoidal variations with periods of 3.337 hrs and 2.018 hrs and other smaller variations have been detected from the period analysis of Hel Λ 6678 spectra obtained by our multi-site campaign for ζ Oph in May, 1993. The resultant periodgram is considerably improved (aliasing free) from those in previous publications. The periodicity may be consistent with previous studies except ambiguities of aliasing. It is surprising that almost all detected periods have a common superperiodicity of about 10.05 hrs. We emphasise the importance of multi-site campaign for the study of line-profile variations (lpv) in early-type stars.

Spectrophotometric observations of the Be star ηCen obtained in May, 1993 show smooth variations of both the stellar and circumstellar Balmer discontinuity (BD) on a time scale of hours. Simultaneous photometry and high-resolution spectroscopy suggest possible correlations with the large-amplitude brightness and line-profile variations.

Short-period variations of high-resolution He I 667.8 nm line profiles obtained on 5 nights are analysed and compared to extensive uvby photometry obtained during 1986 – 1993. The separation of short-period and medium-term light variations was done for 4 seasons. All prewhitened magnitudes can be phased with a single period of 1.2564 d. The radial velocity and the first moment of the He I 667.8 line follows the photometric period with a semi-amplitude of 42 km s−1. The large amplitude is very difficult to understand in terms of current models of non-radial pulsation or star spots.

Results of the preliminary investigation of the monitoring of line-profile variations of the Be star 60 Cyg are reported. At least three apparent absorption or emission bumps moving from blue to red were detected.

We present preliminary results of a time-series analysis on the He II λ4541Å, Si III λλ4552, 4567, 4575Å and Hβ λ486lÅ lines of the late O-type dwarf, HD 93521. The spectra show line-profile variations (lpv) − ‘bumps’ and ‘dips’ − of amplitude ∼ 1% of the continuum, moving systematically redwards through the photospheric absorption lines. The analysis implies non-radial pulsation (NRP) as the origin for the lpv.

Eta Orionis is an interesting multiple system of early type stars. It consists of a visual triple system (A, B, C), a spectroscopic triple system (Aabc) and an eclipsing binary Aab. A is a triple system consisting of a 7.989 day eclipsing binary Aab and a gravitationally bound third component Ac with orbital period of about 9.5 years. All components of the pair AB have spectral types in the range 09-B3 (C. Waelkens & P. Lampens 1988). The radial velocity curve is used to constrain the orbital elements and properties of the binary Aab.

The second component Ab is a B-type star that displays rapid absorption line profile variability. Regular patterns are easily visible in the data.

Practically all published models of β Cephei (β Cep) stars assume a spherical profile, despite the fact that many actual β Cep stars display high rotation velocities. As a result of the centrifugally altered profile of the surface of a rapidly rotating star, the surface area as well as the surface normal of an arbitrary element on the stellar surface will be modified. Numerical integration over the observed surface elements produces the observed luminosity and radial velocity of such a star. The effect of including the centrifugal distortion of a rotating model β Cep star on its observed characteristics was studied for inclination angles (i) of 0°, 22°.5, 45°, 67°.5 and 90° and equatorial rotation speeds of 0, 80, 160, 240, 320 and 400 km/s, for pulsation modes of order l = 0 to 4.