Observations of the pulsations of stars can be used to infer their interior structure and test theoretical models. The main-sequence γ Doradus (Dor) and δ Scuti (Sct) stars with masses 1.2-2.5 M are particularly useful for these studies. The γ Dor stars pulsate in high-order g-modes with periods of order 1 day, driven by convective blocking at the base of their envelope convection zone. The δ Sct stars pulsate in low-order g-and p-modes with periods of order 2 hr, driven by the κ mechanism operating in the He ii ionization zone. Theory predicts an overlap region in the Hertzsprung-Russell diagram between instability regions, where "hybrid" stars pulsating in both types of modes should exist. The two types of modes with properties governed by different portions of the stellar interior provide complementary model constraints. Among the known γ Dor and δ Sct stars, only four have been confirmed as hybrids. Now, analysis of combined Quarter 0 and Quarter 1 Kepler data for hundreds of variable stars shows that the frequency spectra are so rich that there are practically no pure δ Sct or γ Dor pulsators, i.e., essentially all of the stars show frequencies in both the δ Sct and the γ Dor frequency range. A new observational classification scheme is proposed that takes into account the amplitude as well as the frequency and is applied to categorize 234 stars as δ Sct, γ Dor, δ Sct/γ Dor or γ Dor/δ Sct hybrids.

The analysis of the light curves of 48 B‐type stars observed by Kepler is presented. Among these are 15 pulsating stars, all of which show low frequencies, characteristic of slowly pulsating B (SPB) stars. Seven of these stars also show a few weak, isolated high frequencies and they could be considered as SPB/β Cephei (β Cep) hybrids. In all cases, the frequency spectra are quite different from what is seen from ground‐based observations. We suggest that this is because most of the low frequencies are modes of high degree which are predicted to be unstable in models of mid‐B stars. We find that there are non‐pulsating stars within the β Cep and SPB instability strips. Apart from the pulsating stars, we can identify stars with frequency groupings similar to what is seen in Be stars but which are not Be stars. The origin of the groupings is not clear, but may be related to rotation. We find periodic variations in other stars which we attribute to proximity effects in binary systems or possibly rotational modulation. We find no evidence for pulsating stars between the cool edge of the SPB and the hot edge of the δ Sct instability strips. None of the stars shows the broad features which can be attributed to stochastically excited modes as recently proposed. Among our sample of B stars are two chemically peculiar stars, one of which is a HgMn star showing rotational modulation in the light curve.

Abstract. Effects of rotational mode coupling on photometric parameters of stellar oscillations are studied. At moderate rotation rates, a strong coupling between modes of spherical harmonic degree, , differing by 2 and of the same azimuthal order, m, takes place if the frequencies are close. This is a common situation amongst main sequence pulsators. Numerical results for a sequence of β Cephei star models are reported for the two-and three-mode couplings. One consequence of mode coupling is that modes of higher degree should be considered in photometric mode identification. Modes with nominal degree > 2 acquire substantial ≤ 2 components and therefore are more likely to reach detectable amplitudes. Coupled mode positions in the amplitude ratio -phase difference diagrams, based on multicolour photometry, become both aspect-and m-dependent. Examples of the mode path in the diagram with varying aspect are given. The diagrams remain a useful tool for mode identification in rotating stars but the tool must be used with care.

Abstract. In δ Scuti star models, the calculated amplitude ratios and phase differences for multi-colour photometry exhibit a strong dependence on convection. These observables are tools for the determination of the spherical harmonic degree, , of the excited modes. The dependence on convection enters through the complex parameter f , which describes bolometric flux perturbation. We present a method of simultaneous determination of f and harmonic degree from multi-colour data and apply it to three δ Scuti stars. The method indeed works. Determination of appears unique and the inferred values of f are sufficiently accurate to yield a useful constraint on models of stellar convection. Furthermore, the method helps to refine stellar parameters, especially if the identified mode is radial.

We study the frequency distributions of δ Scuti stars observed by the Kepler satellite in short-cadence mode. To minimize errors in the estimated stellar parameters, we divided the instability strip into ten regions and determined the mean frequency distribution in each region. We confirm that the presence of low frequencies is a property of all δ Scuti stars, rendering meaningless the concept of δ Sct/γ Dor hybrids. We obtained the true distribution of equatorial rotational velocities in each region and calculated the frequency distributions predicted by pulsation models, taking into account rotational splitting of the frequencies. We confirm that rotation cannot account for the presence of low frequencies. We calculated a large variety of standard pulsation models with different metal and helium abundances, but were unable to obtain unstable low-frequency modes driven by the κ mechanism in any model. We also constructed models with modified opacities in the envelope. Increasing the opacity at a temperature log T = 5.06 by a factor of two does lead to instability of low-degree modes at low frequencies, but also decreases the frequency range of δ Sct-type pulsations to some extent. We also re-affirm the fact that less than half of the stars in the δ Sct instability strip have pulsations detectable by Kepler. We also point out the huge variety of frequency patterns in stars with roughly similar parameters, suggesting that nonlinearity is an important factor in δ Sct pulsations.

We report a multisite photometric campaign for the β Cephei star 12 Lacertae. 750 h of high‐quality differential photoelectric Strömgren, Johnson and Geneva time‐series photometry were obtained with nine telescopes during 190 nights. Our frequency analysis results in the detection of 23 sinusoidal signals in the light curves. Ten of those correspond to independent pulsation modes, and the remainder are combination frequencies. We find some slow aperiodic variability such as that seemingly present in several β Cephei stars. We perform mode identification from our colour photometry, derive the spherical degree ℓ for the five strongest modes unambiguously and provide constraints on ℓ for the weaker modes. We find a mixture of modes of 0 ≤ℓ≤ 4. In particular, we prove that the previously suspected rotationally split triplet within the modes of 12 Lac consists of modes of different ℓ; their equal frequency splitting must thus be accidental. One of the periodic signals we detected in the light curves is argued to be a linearly stable mode excited to visible amplitude by non‐linear mode coupling via a 2:1 resonance. We also find a low‐frequency signal in the light variations whose physical nature is unclear; it could be a parent or daughter mode resonantly coupled. The remaining combination frequencies are consistent with simple light‐curve distortions. The range of excited pulsation frequencies of 12 Lac may be sufficiently large that it cannot be reproduced by standard models. We suspect that the star has a larger metal abundance in the pulsational driving zone, a hypothesis also capable of explaining the presence of β Cephei stars in the Large Magellanic Cloud.

Abstract. We perform a detailed seismic study of the β Cep star HD 129929. Our analysis is based on the recent derivation of six pulsation frequencies. These frequencies are unambiguously identified from the seismic modelling and the photometric amplitudes to be the radial fundamental, the = 1, p 1 triplet, two consecutive components of the = 2, g 1 quintuplet. A non-adiabatic analysis allows us to constrain the metallicity of the star to Z ∈ [0.016, 0.022]. In addition, the fitting of three independent frequencies, two of which correspond to axisymmetric (m = 0) modes, allows us to constrain the core overshooting parameter to α ov = 0.10 ± 0.05, as well as the other global parameters of the star. Finally, from the observation of the = 1 triplet and part of the = 2 quintuplet, we derive constraints on the internal rotation of this star.

We examine 4-yr almost continuous Kepler photometry of 115 B stars. We find that the light curves of 39 percent of these stars are simply described by a low-frequency sinusoid and its harmonic, usually with variable amplitudes, which we interpret as rotational modulation. A large fraction (28 percent) of B stars might be classified as ellipsoidal variables, but a statistical argument suggests that these are probably rotational variables as well. About 8 percent of the rotational variables have a peculiar periodogram feature which is common among A stars. The physical cause of this is very likely related to rotation. The presence of so many rotating variables indicates the presence of star spots. This suggests that magnetic fields are indeed generated in radiative stellar envelopes. We find five β Cep variables, all of which have low frequencies with relatively large amplitudes. The presence of these frequencies is a puzzle. About half the stars with high frequencies are cooler than the red edge of the β Cep instability strip. These stars do not fit into the general definition of β Cep or SPB variables. We have therefore assumed they are further examples of the anomalous pulsating stars which in the past have been called "Maia" variables. We also examined 300 B stars observed in the K2 Campaign 0 field. We find 11 β Cep/Maia candidates and many SPB variables. For the stars where the effective temperature can be measured, we find at least two further examples of Maia variables.