Using recent and earlier observations of M3 obtained at Konkoly Observatory, and all the other available photometry, the five known double mode RR Lyrae (RRd) stars have been analyzed. Long term amplitude and period changes of both modes have been followed.

We present new data on the period changes of seven RRab stars in the globular cluster M53. The most interesting result of this work is the strong evidence for cyclic period changes for more than half of the studied RR Lyrae variables.

New observations show that RR Lyrae stars in the metal-rich globular clusters NGC 6388 and NGC 6441 have unusually long periods, which indicate that the horizontal branches of these clusters are unusually bright for their metallicities, as predicted by Sweigart & Catelan (1998).

We applied the pulsational method (Caputo 1997) to derive the distances to a sample of galactic globular clusters with well-observed RR Lyrae populations. To apply the method we calculated a set of pulsational theoretical boundaries of the instability strip for the range of masses and chemical compositions spanned by the analysed clusters. In this way we were able to fix simultaneously the apparent distance modulus and the absolute visual magnitude of the RR Lyrae population of each cluster in the sample. As a result we derived the following relations:

We describe a photometric survey for variable stars in twelve metal-rich globular clusters, and provide results for the first two clusters.

The discrepancy between the long distance scale as derived, e.g., from Hipparcos-based distances to globular clusters via main sequence fitting to local subdwarfs, and the short distance scale as derived, e.g., from the absolute magnitude of field RR Lyrae stars via statistical parallaxes and the Baade–Wesselink method, could be accounted for if an intrinsic difference in luminosity of about 0.1−0.2 mag were found to exist between horizontal branch (HB) stars populating the sparse general field and the dense globular clusters.

A study of the period–amplitude relation for RR Lyrae variables in the globular clusters 47 Tucanae, NGC 6441 and M55 confirms a recent finding that the period–amplitude relation for RRab stars with ‘normal’ light curves is not a function of metal abundance. Evolutionary effects are more important.

The relations between the physical parameters of the RRab stars and the Fourier parameters of their light curves have been reanalyzed on the basis of new CCD data. We show that the absolute magnitude MV and the dereddened color indices (B − V)0, (V − I)0 are linear functions of the period and of the A1 Fourier amplitude of the V light curve. Due to the large datasets, we are able to test the consistency of the formulae on independent sets. The present results are in good or fair agreement with our previous studies, despite the fewer number of significant parameters entering in the revised formulae.

For the first time important irregularities in atmospheric pulsations of RR Lyrae itself are clearly detected. They occur from one pulsation cycle to another. Their origins cannot be directly explained by the Blazhko effect. We suggest that these irregularities are mostly provoked by strong shock waves propagating within the atmosphere.

We present results on three presumed “anomalous” RRab stars: one is found to be an RRc variable, and the two others need further study.

We present the first results of a search for variable stars in the Fornax dwarf galaxy.

We present results of a variability survey in the outer-halo globular cluster NGC 6229. Our main goal is to shed light on the origin of the bimodal horizontal branch (HB) of this cluster.

Fourier coefficients for the variable stars in the OGLE survey of the Galactic Center and several globular clusters are presented. The characteristics of the RR Lyrae, δ Scuti and SX Phe stars are presented and a comparison to those observed in other studies is made. In some cases the coefficients of the stars can be used to determine distances and the physical characteristics of the variables.

We show that the spread of the Magellanic Cloud RRd stars in the Petersen diagram may be explained in terms of spread in metallicity by one order of magnitude.

We present the results of follow-up ground-based photometric and spectroscopic observations of a sample of variable stars discovered by the Hipparcos satellite.

Photometric observations of RR Lyrae obtained in the past one hundred years are reviewed and some important results are presented.

We provide the first detection of a frequency multiplet in the line profile variations of RR Lyrae. Performing a period analysis on 669 high resolution line profiles obtained with the spectrograph ELODIE at OHP, we clearly detect a multiplet structure, with a separation equal to the Blazhko frequency, around the main frequency and its harmonics. The triplet components are very prominent; additional observations are needed to decide about the existence of a quintuplet. The complete understanding of the origin of the Blazhko effect still needs further theoretical modelling and better observations. Our detection of the frequency multiplet in the line profile variations is a first essential step towards a decisive confrontation between the theoretical models and the observations.

We present the first results of the analysis of 22 Blazhko stars. We find: 1) Blazhko RRab stars that are nearly pure amplitude modulators; 2) Blazhko RRab stars that have both amplitude and phase modulation; 3) A Blazhko RRab star that has an abrupt period change; 4) Proof of the Blazhko effect in RRc stars. Our data show the character of the amplitude and phase modulations of the light curves over the Blazhko cycles far better than has been previously possible.

By assuming that RR Lyrae stars have fairly strong dipole magnetic fields with symmetry axis oblique to the rotation axis of the star, I show that the oscillation mode which would be a pure radial oscillation in absence of the magnetic field has a quadrupole component, whose symmetry axis coincides with the magnetic axis. The aspect angle of the quadrupole component changes due to the stellar rotation, and this apparent amplitude variation is interpreted as the Blazhko effect in RR Lyrae stars. It is shown that, in the case of off-axis magnetic field, phase modulation is expected. I list up some theoretical predictions based on this model, which would observationally examine this hypothesis.

With about one fourth of the RRab stars showing the Blazhko effect, amplitude modulation on long time scales is a common phenomenon in stellar pulsation. The role of nonradial oscillation modes is studied here, and it is proposed that the amplitude modulation is a result of a dynamical, nonlinear resonance between the observed radial mode and a low-degree nonradial mode. In this scenario, a nonradial mode is resonantly excited in Blazhko-type RR Lyrae stars. Degree one modes are found to have the highest probability of being excited and a crucial test of the model would be the observational determination of the degree of the proposed nonradial mode.