Alexandru Pop

On the period variability of GP andromedae

The present paper deals with the pulsation period variability of the high-amplitude δ Scuti star GP Andromedae using a data set covering a time base of 26.9 years. The possibility of a periodic component (reflecting the light-time effect induced by a hypothetic binarity) in the O-C curve is inferred. The presence of this periodicity in the period variation of GP And leads to a diminution of the discrepancy between predicted and observed relative period change rates. The hypothesis of the existence of an unseen companion is discussed. The statistical analysis of the final O-C residuals distribution, indicates their normal or close to normal character.

DETERMINISTIC COMPONENTS IN THE LIGHT CURVE AMPLITUDE OF Y OPH

About two decades after the discovery of the amplitude decline of the light curve of the classical Cepheid Y Oph, its study is resumed using an increased amount of homogenized data and an extended time base. In our approach, the investigation of different time series concerning the light curve amplitude of Y Oph is not only the reason for the present study, but also a stimulus for developing a coherent methodology for studying long- and short-term variability phenomena in variable stars, taking into account the details of concrete observing conditions: amount of data, data sampling, time base, and individual errors of observational data. The statistical significance of this decreasing trend was estimated by assuming its linearity. We approached the decision-making process by formulating adequate null and alternative hypotheses, and testing the value of the regression line slope for different data sets via Monte Carlo simulations. A variability analysis, through various methods, of the original data and of the residuals obtained after removing the linear trend was performed. We also proposed a new statistical test, based on amplitude spectrum analysis and Monte Carlo simulations, intended to evaluate how detectible is a given (linear) trend in well-defined observing conditions: the trend detection probability. The main conclusion of our study on Y Oph is that, even if the false alarm probability is low enough to consider the decreasing trend to be statistically significant, the available data do not allow us to obtain a reasonably powerful test. We are able to confirm the light curve amplitude decline, and the order of magnitude of its slope with a better statistical substantiation. According to the obtained values of the trend detection probability, it seems that the trend we are dealing with is marked by a low detectibility. Our attempt to find signs of possible variability phenomena at shorter timescales ended by emphasizing the relative constancy of our data, within their precision limits.

Y Leonis reloaded

The aim of the present contribution is to stimulate the interest in observational and theoretical study of the Algol type binary system Y Leonis. Different arguments are presented both from the viewpoint of stellar photometry and orbital period variability. Preliminary observational results are also presented.

The variability of the pulsation period of DL Cassiopeiae

The temporal behaviour of the pulsation period of the classical Cepheid DL Cas was reconsidered using new available photoelectric data. Apparently the pulsation period is constant, but we could expect that possible evolutionary changes should still be hidden in observational noise as well as the light‐time effect. Therefore, the investigation of the period variability requires additional observational data with better S/N ratio and phase coverage.

Double‐mode pulsations of RZ Cephei?

On the basis of old observational data as well as our own CCD observations, we emphasized strong evidences for the existence of additional variability in the light curve of RZ Cephei, at time scale of about 0.8d. The variability of the nearby star TYC 4273‐659‐1 is also inferred.

Light curve shape variability of RZ Cephei. Preliminary considerarions

In spite of the existence of some observational evidences on the variability of the light curve shape of the RRc star RZ Cephei, our analysis seem to indicate its stability. This conclusion may not be however, the final one, because of the low S/N ratio of the most available data and the possible pathological aspects existing in some of them. New high precision and high sampling rate observations are needed in order to give a verdict in the question light curve shape variability versus stability.

Light curve shape variability of SZ cassiopeiae between 1978–1994

The aim of our investigation was toapproach the problem of identification for possiblevariability of light curve shape in case of theclassical Cepheid SZ Cassiopeiae. In order to performsuch a study we analysed eight observational data setsobtained through V photoelectrical photometry,covering a time base of 16 years. We applied theFourier decomposition technique to each data set; theobtained Fourier coefficients were used to calculatethe corresponding amplitude ratios and phasedifferences. Their time behavior were finally studiedin order to search for evidences of the variability ofthe light curve shape.

Structural Properties of Pulsating Star Light Curves Through Fuzzy Divisive Hierarchical Clustering

The aim of this paper is to explore the possibility of using a fuzzy divisive clustering technique to the classification of pulsating stars according to the shape of their light curves. The considered sample contains Cepheides, RR Lyrae stars, and high amplitude δ Scuti and SX Phoenicis stars. Our investigations proved the ability of this method to identify morphological groups in a given class of pulsating stars. The method is also useful for establishing the membership of an unknown variable star in a given class.

The study of binary systems with pulsating components using the frequency-modulation theory

The effect of the orbital motion on the light curve of a pulsating star has been evaluated by means of the amplitude and phase spectra synthesis based on the instantaneous pulsation phase concept. The role of the binary systems geometry (circular or eccentric orbit) and the possibility of binarity diagnosis are also emphasized.