We summarise the history of the λ Boo criterion and our strategy to determine the evolutionary status of this group of stars.

We present fine analysis results for roAp and λ Boo stars.

For an optimum grid we postulate a fractional change of the emergent Stokes parameters between two adjacent surface points not to exceed some fixed value (say 20 %) at any frequency point. The contribution from a local Stokes IQUV profile to the observed integrated profile is a complicated non-linear function of rotational and pulsational Doppler shifts respectively, of Zeeman splitting, magnetic field direction relative to the line-of-sight, of the magnetic field azimuth in the observer’s frame and of limb darkening. The run of IQUV with visible disk coordinates (x, y) can be represented by curved surfaces f(x,y); these are different for each Stokes parameter and frequency point. The optimum integration grid then has to ensure that the maximum change δf(x,y) over all frequencies and Stokes parameters does not exceed the above-mentioned limit.

The present work deals with the effect of rotation on the adiabatic oscillation frequencies of stellar models. Here, rotation, as described in a perturbation theory, is said to be rapid in the sense that second order perturbation effects have to be taken into account. Rotation has two second order effects on the oscillation frequencies: departure from equidistance within a given multiplet split by rotation and an overall shift of the multiplet itself. The first effect has been investigated for realistic models of δ Scuti stars in uniform rotation (Dziembowski & Goode, 1992), whereas H. Saio’s study includes both effects for a polytrope of index 3 in uniform rotation (Saio 1981). Both effects are investigated for realistic models of δ Scuti stars. Results for the overall shift for a 2 M⊙ model in rapid uniform rotation (v up to ∼ 200 km/s) are outlined below. The rotation rate corresponding to this surface velocity is Ω = 1.44 10−4 rad/s.

28 Aquilae (HR 7331, V1208 Aql) was first announced as a δ Scuti type variable by Breger in 1969 with a period of about 0.15 day. It was clear that the light variability could not be adequately represented as a sinusoidal variation with just one frequency. Since then several observers have provided additional light curves and new analyses of the light variation. In this paper we discuss observations which we have made and an analysis of the data available since 1966.

Our objective was to find the frequencies of components which would represent the light variation of 28 Aql over the 28 years for which the star has been observed.

Using the 30 cm reflector of the Hume Cronyn Memorial Observatory we observed 28 Aql in 1979, 1980, 1981, and 1993 in blue light. The light curves for JD 2444140 and 2449190 are shown below. Comparison stars were HD 180868 and HD 181383. Magnitude differences are referred to HD 181383. Nights on which the scatter in the observations, due to poor transparency, appeared to be excessive were discarded. Observations on 21 nights were retained for analysis.

Six δ Scuti stars have been detected in the open cluster NGC 6134 (Frandsen et al. 1995). A first mode identification attempt shows that several radial modes of radial order n = 0, 1, 2 or 3 might have been observed. We may also be dealing with g-modes and/or modes split by rotation.

Within two groups or networks we are trying to build the observational fundamentals for better tests of stellar evolution. The first group is STACC (Small Telescope Array with CCD Cameras) and the second the EU HCM program ANTENA (involving institutes in Spain, France and Denmark).

The first information needed is photometric data. Colour-Luminosity diagrams and time series photometry must be obtained. The time series require extensive campaigns in order to give the high quality data needed for asteroseismology. Ideally one should have time series of several quantities (see Balona & Stobie 1979, Mantegazza et al. 1994).

The second type of observations is of spectroscopic nature. It is needed to ascertain membership of the stars in a given cluster and to check binarity. Also rotational velocities must be observed to help the attempt to identify the modes in 6 Scuti stars. Finally some information about metallicity and peculiarity is required.

It has been pointed out by earlier authors that radial pulsation seems to be the only one type present in high amplitude δ Set stars, at least for monoperiodic stars. In order to confirm or not this point, we have collected all the multicolour data available for these stars in the Strömgren and Johnson photometric systems. Then, the type of pulsation has been analysed on the basis of the phase shifts and amplitude ratios between observed light and colour variations. The results indicate that all the stars analysed are radial pulsators.

The variability of CD-24 7599 (V=11.48 mag) was discovered by JCC during observing run XCOV7 of the Whole Earth Telescope (WET, Nather et al. 1990) network in February, 1992. The star was observed as an additional target and 117 hours of high-quality temporal spectroscopic observations were obtained.

Our analysis of these data revealed the presence of 7 independent pulsation modes between 27.0 and 38.1 cycles per day (313 – 441 μHz) with semiamplitudes of 2.1 – 10.2 milli-modulation amplitudes (mma). We showed that peaks at linear combination frequencies detected in the power spectra were not due to eigenmodes excited to visible amplitude by resonant mode coupling.

On the 1992 STEPHI IV campaign, the multi-periodic δ Scuti stars BN Cnc and BU Cnc of the Praesepe cluster were monitored during a three-week, three-continental run (Belmonte et al. 1994). Five and six frequency peaks, respectively, were recorded as a result of the campaign. In a subsequent paper (Pérez Hernández et al. 1995, hereafter Paper I), an attempt to identify these frequency peaks as pulsation modes was done. An important point is that both stars belong to the same cluster and, hence, more severe constraints than for isolated stars can be imposed. Recently, Breger et al. (1994) have reported results on EP Cnc, another δ Scuti in Praesepe. This star is also multi-periodic, showing three frequency peaks above the noise level. In the present work we extend our analysis to this star.

The high stellar densities at the centre of some globular clusters provide prolific breeding grounds for exotic stars, e.g., the 11 msec pulsars in 47 Tuc and numerous blue stragglers. The same high density of stars as viewed on the sky makes ground-based photometry problematic for variable star detection. Even a very few tight binaries can be of fundamental importance for the dynamic evolution of the cluster core. We present results of a continuous 40-hour sequence of U-band CCD exposures acquired with WF/PC on HST. The high spatial resolution coupled with a stable PSF allows nearly Poisson limited time series on some 20,000 stars. The 1000 second sampling and long coverage provide excellent sensitivity to W UMa and δ Scuti variations. Light curves are shown for selected variables and the relative frequencies of binary and variable stars are discussed.

Extensive photometric monitoring of bright δ Scuti stars, made in the last years, allowed us to get reliable frequency determinations also in very complicated cases. Table 1 gives a full description of our photometric targets. Recently, we realized that combining photometry with simultaneous high resolution spectroscopy, in order to study line profile variations, offers the possibility to perform a much more reliable mode identification. Hence, the photometric runs of FG Vir, X Cae, HD 2724 were paired with spectroscopic ones obtained at the Coudé Auxiliary Telescope (ESO, La Silla, Chile). The stellar spectra are centred at 4508 Å and they cover a range of 37.6 Å; the resolving power is about 50000-60000 and the linear dispersion is 2.4 Å/mm.

The stars HD 224638 and HD 224945 belong to the new class of F0 V spectral type stars which show an unusual kind of variability for that region of the HR diagram (Mantegazza et al. 1993).

The variability of these two stars has been discovered and studied by Mantegazza, Poretti & Zerbi (1994). They show small amplitude light variations with characteristic time scales of the order of one day. The light curves are not periodic and it is difficult to satisfactorily fit them even with several periodic terms.

In order to explain our 1991 B colour observations we tentatively suggested that in both stars two close periodic terms were present with a characteristic frequency of about 0.8 c/d, and that these terms had a double wave shape for HD 224638 and a triple wave one for HD 224945.

A third periodicity is found in the early F-type star γ Dor which probably rules out an interpretation in terms of differential rotation of a spotted star. One of the two periods in 9 Aur is not visible in the radial velocity. This is not explained by the star spot model, but may indicate that the two periodicities are nonradial pulsations of different spherical harmonic degrees.

We address the problem of the preferential mode excitation in magnetic Ap-stars by estimating the pulsation energy losses in the atmosphere through the generation of the Alfvenic waves.

The Cape rapidly oscillating Ap star survey is a systematic search for rapidly oscillating Ap stars in the southern hemisphere. To date, 12 new roAp stars have been discovered. This paper describes the current state of the survey.

We present a pulsational RV curve for γ Equ whose amplitude is about 200 m/s and period is roughly 11 minutes. These data, combined with rapid photometry and line-profile variations, are being used to identify the nonradial mode(s) and study mode growth/decay in this unusual star.

Since 1981, we have been regularly carrying out photoelectric observations of Cepheids. By now nearly 20450 observations in UBVRI have been acquired. These observations are the basis of our Cepheid data base; presently we have collected on magnetic media near 88000 photoelectric as well as near 18000 radial velocity measurements. We shall continue to append to this Cepheid data base our new observations as well as published data.

A relation between the Fourier decomposition parameters of Cepheid light curves and metallicities in the form [Fe/H]=f({Ri1, ρi1}) is presented. It gives [Fe/H] values with prediction errors 0.16−0.22 dex depending on the number of Fourier parameters used in the relation.

The main aim of this work is to consider the evolutionary status of s-Cepheids (SC), - small-amplitude Cepheids with smooth sinusoidal light, colour and radial velocity curves. The unusual character of these objects could be explained in two ways: (1) SCs are crossing the Cepheid instability strip for the first time and they are first (or high) overtone pulsators (Efremov 1970); (2) SCs have close early-type companions, which affect gravitationally the Cepheid He II zone. Such an effect can decrease the Cepheid pulsational activity (Lloyd Evans, 1968).

According to theoretical calculations, low helium abundance in Cepheid atmospheres, which are crossing the instability strip for the first time, could produce small-amplitude oscillations. On the other hand, double-mode Cepheids (DMC) show oscillations with two (or three) periods: fundamental with large-amplitude asymmetrical light, colour and radial velocity curves, and first overtone with smooth sinusoidal small-amplitude ones. Nevertheless, detailed spectral and photometric investigations permit the detection of companions for the brightest SCs, and support the second hypothesis.