For shell hydrogen-burning δ Scuti stars, many more unstable modes are predicted than are observed. For example, for 4 CVn and δ Scuti itself, only 18 and 6 modes are observed, respectively, whereas several hundred ℓ = 0, 1 and 2 rotationally-split modes are predicted. The predicted modes have a mixed p- and g-mode character, with many g-type nodes present in the H-exhausted core and shell H-burning region, where the Brunt-Väisälä frequency is large.

Here we explore whether the predicted frequency spectrum can be made to agree better with the observed spectrum if the g-mode character of the pulsation modes is partially suppressed. Additional motivation for this approach is provided by the observation by Breger et al. (1999) that 4 modes in 4 CVn identified as ℓ = 1 have nearly equal frequency spacings of ∼14 μHz, reminiscent of consecutive radial orders from a pure p-mode spectrum. We consider 2.1- and 2.3-M⊙ evolutionary models that match the observed Teff, L, and log g, plus an identified radial mode frequency of 4 CVn and δ Scuti. For these models, we experimented with setting the Brunt-Väisälä frequency to zero from the model center to the edge of or beyond H-depleted core, so that the g-type portion of the waves becomes evanescent in this region.

We suggest that the modulation of radiative flux by convection at the base of a deep envelope convection zone during the pulsation cycle is responsible for driving high-order g modes with periods 0.4 to 3 d observed in γ Doradus variables.

We investigate a sequence of 1.5 M⊙ models for g-mode instabilities. For the computation of the nonradial, non-adiabatic oscillations we make use of the Riccati method introduced by Glatzel & Gautschy (1992) to integrate the pulsation equations as given by Unno et al. (1989).

A search for new γ Dor stars was undertaken using the Hipparcos periodic variable star catalogue and the Geneva photometric database, leading to a list of 40 new candidates. We started a monitoring of the candidates which suited the observational window with the CORALIE spectrograph at the Swiss Euler Telescope for establishing a robust list of new γ Dor stars and studying line profile variations. We present here our long-term program.

A code for solving non-adiabatic radial pulsation equations with consistent treatment of radiative transfer is used to study the oscillations in the atmospheres of the rapidly oscillating Ap stars. We find that our results are consistent with the presence of an observable node in the temperature eigenfunction.

In the present work we develop a theoretical model for roAp stars characterized by the suppression of convection around the magnetic poles. When calculating the growth rates of acoustic oscillations in models of this type we find that most models whose positions in the HR diagram coincide with that of the observed roAp stars are unstable against high-order pulsations.

We present the echelle-diagrams for p-mode spectra of roAp stars HD 60435, HR 1217 and γ Equ and discuss some their peculiarities.

A time-series CCD photometric survey of variable stars in open clusters has been in progress using 1.8-m telescope at the Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. We present preliminary results of time series photometry for the intermediate-age open cluster NGC 2301.

We present some preliminary results of a Strömgren photometric survey searching for radially pulsating variables among Herbig Ae stars. These young objects are expected to show a characteristic δ Scuti variability when entering the instability strip along their pre-main sequence evolutionary tracks, according to the theoretical predictions by Marconi & Palla (1998). We identified at least five pulsating Herbig Ae stars with periods in the expected range. On the basis of these encouraging first results, we foresee both new more detailed observations and accurate comparisons with theoretical models.

The aim of this paper (further developed in Barban et al. 1999) is to present new evidence of the possible stellar origin of the observed excess power in the power spectrum of Procyon A presented in Martic et al. (1999) by comparing these observational data with theoretical predictions and numerical simulations.

δ Sct stars are among the most promising targets to perform ground-based asteroseismology. High resolution spectroscopy offers us a powerful technique to identify radial and nonradial pulsation modes, since we can easily detect oscillations and travelling features in the line profiles.

There are many advantages to studying variable stars in open clusters; the parameters that determine the evolution of the stars, such as chemical composition and age, can be considered to be the same for all the stars in the cluster. These data, along with the mass, effective temperature and superficial gravity, allow us to better determine the physics which explain the pulsation mechanisms. In the present paper the variable stars h501 and h906 in α Per and w2 and w20 in NGC 2264 are studied. In particular, the reasons why δ Scuti stars coexist with early type stars in NGC 2264 is examined.

HD 200925 is an unusual star: the period is stable on a time scale of many years, yet it shows variations which are either large and irregular, or small, suggesting a possible Blazhko effect (Poretti 1984; Joner & Johnson 1985); it has larger values of Teff and log g derived from uvbyβ than those of dwarf Cepheids of similar period; and it has a somewhat higher metal abundance, [Fe/H], than the Hyades. Furthermore, in conflict to what Joner and McNamara (1983) determined – that m1 index shows no variation with temperature – HD 200925 shows a definite variation, unlike that predicted in Crawford (1979). No explanation of this has yet been found. Johnson & Joner (1986) also derived an unusual and unexpected value of [Fe/H]. Finally, with respect to the pulsation periods of this star, Mantegazza & Poretti (1986) determined the existence of two periods: one definite, 0.26730 d and a possible second one of 0.2138 d which yield a ratio of P2/P1 = 0.800, the theoretical ratio expected between the first and the second overtones, found in other pulsating variables. All of these facts make HD 200925 an interesting star, motivating the present study.

We present the first results from multi-site observations of the δ Scuti star XX Pyx (CD–24°7599). The observations were carried out as the 17th run of the Delta Scuti Network. We collected 583 hr of B, V time-series photometry, resulting in a detection level (4σ) in the amplitude spectrum of 0.5 mmag. We detect 6 new pulsation frequencies, bringing the total number of frequencies known in this star up 19.

The STACC campaign of 1998 on BN Cnc was an extensive international photometric campaign aimed at obtaining accurate frequencies of the pulsations in this star. To facilitate identification of the modes, parallel spectroscopic observations were made from six observatories around the world. I here present preliminary results from the spectroscopy made from the Nordic Optical Telescope. The accurate frequencies from the photometric campaign are used to obtain accurate amplitudes of the equivalent width variations. These results are to be used in an attempt to identify the pulsation modes in this star.

This is a preliminary report on the CCD (mainly) photometry of the two δ Scuti stars, BN Cnc and BV Cnc, in the Praesepe open cluster.

Some new times of light maxima for three δ Scuti stars – CY Aqr, BS Aqr and AD CMi – have been determined based on new photoelectric photometry. Incorporated with the times presented in the literature, the O–C method is used to study the period variations of these stars with a newly-developed program OMC. The results reveal that for these three stars, the binary hypothesis concerning a low mass companion is reasonable in explaining the O–C diagrams.

The δ Scuti stars are pulsating variables located in the lower part of the Cepheid instability strip with spectral types from A2 to F0 on the main sequence, and from A3 to F5 at luminosity class III. These variables show short periods (< 0.3day) and luminosity amplitudes ranging from a few thousandths of a magnitude to several tenths. Over the last few years, significant progress has been made in the detection of pulsating modes in the framework of the multisite campaigns, e.g. STACC (Frandsen et al. 1996), DSN (Breger et al. 1998), STEPHI (Michel et al. 2000). For the 1998 STEPHI IX photometry campaign, the δ Scuti star V534 Tau of the Pleiades cluster (see Table 1) was monitored during a three week, three continent run. Preliminary results are reported here.

Five nights of photoelectric photometric observations through v and y filters confirm the pulsation of the Am δ Scuti variable 60 Tau. Power spectra of the light curves show multiple periods; two modes f1 = 13.0364 d−1 and f2 = 11.8521 d−1 are definitely identified. The phase differences, ϕv − ϕy, suggest that 60 Tau pulsates in low-degree modes; the pulsation constants, Q, suggest these are low overtone modes; and the frequency ratio implies that at least one mode is nonradial.

V647 Tau was observed during a 1997 STEPHI campaign; 228 hr of observations in v were obtained, together with another 50 hr of simultaneous data in y. The data were analyzed and main results are discussed in the text.