New observations of V534 Tau were performed during the STEPHI IX campaign in November 1998. An overall run of 285 h of data has been collected from three sites over a period of 23 days. Period analysis reveals a rich oscillation spectrum with nine frequencies above the 99% confidence level. This oscillation spectrum spans a large range of frequencies, from 179 to 525 muHz (15.52 to 45.36 cycles per day). A preliminary comparison with models suggests that the observed modes would lie in the range of modes from g(1) or g(2) to P-4 or p(5), depending on the rotation rate considered for V534 Tau.

The pulsating PG 1159 planetary nebula central star RXJ 2117+3412 has been observed over three successive seasons of a multisite photometric campaign. The asteroseismological analysis of the data, based on the 37 identified l = 1 modes among the 48 independent pulsation frequencies detected in the power spectrum, leads to the derivation of the rotational splitting, the period spacing and the mode trapping cycle and amplitude, from which a number of fundamental parameters can be deduced. The average rotation period is 1.16 +/- 0.05 days. The trend for the rotational splitting to decrease with increasing periods is incompatible with a solid body rotation. The total mass is 0.56(-0.04)(+0.02) M. and the He-rich envelope mass fraction is in the range 0.013-0.078 M-*. The luminosity derived from asteroseismology is log(L/L.) = 4.05(-0.32)(+0.23) and the distance 760(-235)(+230) pc. At such a distance, the linear size of the planetary nebulae is 2.9 +/- 0.9 pc. The role of mass loss on the excitation mechanism and its consequence on the amplitude variations is discussed.

The evolvedScuti variable 4 CVn was observed photometrically for 53 nights on three continents. We found a total of 34 significant and 1 probable simultaneously excited frequencies. Of these, 16 can be identified as linear combina- tions of other frequencies. All significant frequencies outside the 4.5 to 10 cd 1 (52 to 116 µHz) range can be identified with frequency combinations, fi ± fj, of other modes with gener- ally high amplitudes. There exists a number of closely spaced frequencies with separations of � 0.06 cd 1 . This cannot be explained in terms of amplitude variability. The results show that even for stars on and above the main sequence other than the Sun, a very large number of simultane- ously excited nonradial oscillations can be detected by conven- tional means. Since all pulsation modes with photometric amplitudes of 1 mmag or larger have now been detected for this star, a presently unknown mode selection mechanism must exist to select be- tween the 1000+ of low-degree modes predicted to be excited for this (and many other) stars. Phase differences and amplitude ratios between the y and v colors are determined for the ten main modes. The phase differences indicate p1 to p4 modes of ` = 1 for four of these modes. The formulae to determine the uncertainties in the ampli- tudes and phases of sinusoidal fits to observational data are derived in the Appendix.

A multifrequency analysis of the SX Phoenicis star BL Camelopardalis is presented on the basis of new high-speed photometry, along with fitting a total of 136 maxima. BL Cam is a multiple periodic pulsaton We find f(0) = 25.5768, f(1) = 25.2982, f(2) = 25.8622, f(3) = 31.5912, f(4) = 25.1.065, f(5) = 25.5147 and f(6) = 25.6188 cycled(-1) together with the harmonics 51.1513 and 76.7268 cycled(-1) and combination frequencies f(0) + f(1), f(0) + f(2) and f(0) + f(3). The new frequency solution represents the light curves of BL Cam quite well. The observed minus calculated (O-C) analysis indicates that the fundamental frequency is in good agreement with the results of Fourier analysis.

After the "O - C" technique is used to study the pulsation of the delta Scuti star VZ Cnc with the times of maximum light listed in the literature and a newly-determined time based on new photometry for this star, some high-quality photometry Light curves in the literature are analysed by the Fourier decomposition, showing that there are more than 2 frequencies in the pulsation of this star, which might be one possible explanation of the existence of the large scatter in the "O - C" diagram.

The delta Scuti variable star HR 5437 was observed with the 85 cm telescope at the Xinglong Station of Beijing Astronomical Observatory from 1997 to 1998. Period analyses were made for our data set, the data set obtained by Li and Jiang in 1991 and the combined data set (1991-1998). From the combined data set two frequencies were obtained: 11.3484 and 6.9840 cd(-1). We calculated the evolution sequences of a star with 1.0 - 2.0 solar mass and all pulsation modes of each evolution phase. Among all the modes calculated from the 140th evolution phase of a star with 1.90 solar mass we found two frequencies which are almost the same as the two observed frequencies. So, we tend to suggest that HR 5437 pulsates in two modes: a radial p mode with l = 0, n = 4 and another nonradial p modes with l = 1, n = 1.

New pulsation modes in two delta Scuti stars of Praesepe cluster, BQ and BW Cnc, have been detected during the STEPHI VI campaign in 1995. In particular, 3 frequencies for BQ Cnc and 9 frequencies for BW Cnc have been found above a 99% confidence level. The possibility of the presence of a g-mode present in BQ Cnc is discussed, considering its binarity. The effect of mutual interference between very close detected frequencies in BW Cnc during the observations, is also considered. This last effect reveals the necessity of long-period observing runs, in order to avoid its influence in the final number of detected modes. In such situation, studies of secular amplitude changes can be strongly affected.

Abstract. HD 163151 is discovered to be a new variable as apart of a W UMa system. In this paper we present completeuvby light curves. Additional Crawford H β and Johnson BVdata are also obtained. A detailed photometric analysis basedon these observations is presented using the WD code. Severaltests have been carried out concerning mass ratio and effectivetemperatures. The best fitting was achieved with q = 0.27 andT 1 =6450 K (fixed) and T 2 = 6130 K. Moreover, the light curvesshow evidences of spots in one of the components.Key words: binaries: visual – stars: individual: HD 163151 –stars: fundamental parameters – techniques: photometric1. IntroductionDuring the course of a photometric and multicolor multisitecampaignontheγDorvariableHD164615,thestarHD163151(HR 6676, BD +11 ◦ 3283) was used as a check star, but itsdifferential photometry was discovered to be variable showinglightcurvessimilartothoseofclosebinarysystems,inparticulara W UMa system, with a period of about 0. d 8 and luminosityvariation in the primary minimum of about 0.

New observations of BS and BT Cnc were performed during the STEPHI VII campaign in 1996 February. An overall run of 115 hours of data was collected. Different methods have been used to analyse the time series, bringing the detection of 2 frequencies for BT Cnc and 3 frequencies for BS Cnc above the 99% confidence level. A comparison with the literature reveals that the dominant mode of BT Cnc (similar to 9.8 c/d) has kept excited over tens of years, while the secondary modes have not been the same all the time. However, during the very last years the two frequencies detected by us have been the only ones noticeably excited in the spectrum.

We present in this paper the results from a multisite photometric campaign devoted to the gamma Dor variable HD 164615. During the campaign data were collected in the Johnson and Stromgren photometric systems. A first search for sinusoidal periodicities yielded a three-term solution with frequencies f(1) = 1.2328, f(2) = 1.0899 and f(3) = 2.3501 d(-1), and a long-term trend f(4) = 0.1301 d(-1). The star may therefore be multiperiodic with each frequency related to a pulsation mode. However, a further examination of the curve provided evidence that the amplitude at the instant of maximum brightness is modulated. Alternatively, HD 164615 may be a monoperiodic variable with frequency f(1) = 1.2321 d(-1), showing amplitude modulation with frequency f(4) = 0.1301 d(-1). The presence in the power spectra of the coupling terms f(2) = f(1) - f(4) and f(3) = f(1) + f(4) - 1 supports the latter interpretation. In the monoperiodic case the previously given interpretation based on a spot carried through the visible disc by rotation still holds, provided that the spot is bright and evolves on a time-scale of a few days. However, similarities with other gamma Dor stars provide indirect indication in favour of the existence of a non-radial pulsation interpretation.

A coordinated photometric campaign at five observatories to monitor the suspected brightness variations of the early-l; star HD 108100 was undertaken by the DSN (Delta Scuti Network) and WET (Whole Earth Telescope). 288 hours of high-quality observations during a 58 day time span led to the discovery of two frequencies of 1.32 and 1.40 c/d with V amplitudes of 0.010 and 0.007 mag. The detection of both frequencies is statistically significant with amplitude signal/noise ratios of 18 and 12. The light variations are accompanied by v - y color variations of 0.005 and 0.003 mag, respectively.The variability of HD 108100 is typical of the new class of gamma Dor variables. We also derive amplitude ratios and phase differences between the light curves at different wavelengths. These are consistent with g-mode pulsation of l = 1, but not l = 2. The observed periods can be fit by two modes with successive radial orders of n similar to 19,Studies of the delta Scuti star 4 CVn from 1970 to 1976 reported the existence of unusually low frequencies of 1.3 and 1.4 c/d. These frequencies can now be traced back to HD 108100, the single comparison star used in these studies. Consequently, 4 CVn should no longer be regarded as an example of a star exhibiting both delta Scuti type pulsation (acoustic modes of mixed character) and gamma Dor variability (gravity modes of high order).An analysis of unpublished data from 1970 to 1976 shows that the amplitudes associated with the two frequencies of HD 108100 are variable and that the two frequencies can appear to switch their power.

A criterion for the occurrence of dynamical instability in the convective envelopes of red giants and AGE stars is presented. Using this criterion we find that the effect of turbulent pressure causes the dynamical instability in the regions very close to the surfaces of red giants and AGE stars, which affect the mass ejection outwards. This may be the mechanism powering the strong stellar winds of red giants and AGE stars with very low surface temperature and faint radiation pressure.

— This paper reports on the fifth STEPHI multi-site photometric campaign in 1993 devoted to the δ Scuti star BDS 1269A (VW Ari). The observation yielded a data set of 150.7 hours. Our analysis has resulted in 7 identified frequencies. These frequencies exhibit three couple-frequency character, and also fit the previous investigations quite well. An attempt to identify the pulsation modes is made, but no equidistant evidence can be detected, and a secondorder effect for rotational splitting has to be considered to explain the complicated nature of BDS 1269A.

This paper reports on the fifth STEPHI multi-site photometric campaign in 1993 devoted to the δ Scuti star BDS 1269A (VW Ari). The observation yielded a data set of 150.7 hours. Our analysis has resulted in 7 identified frequencies. These frequencies exhibit three couple-frequency character, and also fit the previous investigations quite well. An attempt to identify the pulsation modes is made, but no equidistant evidence can be detected, and a second-order effect for rotational splitting has to be considered to explain the complicated nature of BDS 1269A.

This paper reports on the fifth STEPHI multi-site photometric campaign in 1993 devoted to the delta Scuti star BDS 1269A (VW Ari). The observation yielded a data set of 150.7 hours. Our analysis has resulted in 7 identified frequencies. These frequencies exhibit three couple-frequency character, and also fit the previous investigations quite well. An attempt to identify the pulsation modes is made, but no equidistant evidence can be detected, and a second-order effect for rotational splitting has to be considered to explain the complicated nature of BDS 1269A.

A coordinated photometric campaign of FG Vir at nine observatories covering 170 hours was undertaken by DSN (Delta Scuti Network) and WET (Whole Earth Telescope). Two different observing techniques were adopted for the two telescope networks in order to optimize different frequency ranges.Ten pulsation frequencies between 9.19 and 34.12 c/d (112 and 395 mu Hz) were detected with amplitudes ranging from 0.8 to 22 mmag. Pulsational instability is observed only in specific frequency regions. Additional frequencies of pulsation within these regions probably exist, but do not reach the significance criterion of amplitude signal/noise adopted by us, Comparisons with previously obtained data show that the amplitudes of the main frequencies are stable over a year or longer.A preliminary identification of the ten dominant frequencies is proposed in a stellar model with 1.8M. in advanced main-sequence phase of evolution. The frequencies correspond to low order p and g modes with l less than or equal to 2 and radial order 1 to 6. According to the linear nonadiabatic calculations, the identified modes are driven by the opacity mechanism along with many other modes. For asteroseismology of delta Scuti stars, FG Vir is an extremely important candidate, especially because of the probable presence of g modes.