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We document the presence of a few Cepheid and RR Lyrae variable stars with previously unrecognized characteristics. These stars exhibit the property of a period ratio of main pulsation divided by secondary pulsation P1/P2 very close to…
We present the results of a spectroscopic follow-up of various puzzling variable objects detected in the OGLE-III Galactic disk and bulge fields. The sample includes mainly short-period multi-mode pulsating stars that could not have been…
We performed a new and accurate fit of light and radial velocity curves of the Large Magellanic Cloud (LMC) Cepheid --OGLE-LMC-CEP-0227-- belonging to a detached double-lined eclipsing binary system. We computed several sets of nonlinear,…
We present new Period-Age (PA) and Period-Age-Color (PAC) relations for fundamental and first overtone classical Cepheids. Current predictions rely on homogeneous sets of evolutionary and pulsation models covering a broad range of stellar…
Classical Cepheids (DCEPs) are important astrophysical objects not only as standard candles for the determination of the cosmic distance ladder but also as a test-bed for the stellar evolution theory, thanks to the connection between their…
Classical Cepheids are blue loop stars that have famously been dubbed "magnifying glasses of stellar evolution" and have been studied for a long time. As more and more precise observations of Cepheids are secured over ever-increasing…
Masses of classical Cepheids of 3 to 11 M$\odot$ are predicted by theory but those measured, clump between 3.6 and 5 M$\odot$. As a result, their mass-luminosity relation is poorly constrained, impeding our understanding of basic stellar…
We report the discovery of a new subclass of double-mode RR Lyrae stars in the Large and Small Magellanic Clouds. The sample of 22 pulsating stars have been extracted from the latest edition of the OGLE collection of RR Lyrae variables in…
Classical Cepheids (CCs) have long been considered excellent tracers of the chemical evolution of the Milky Way's young disk. We present a homogeneous, NLTE spectroscopic analysis of 401 Galactic CCs, based on 1,351 high-resolution optical…
JHKs magnitudes corrected to mean intensity are estimated for LMC type II Cepheids in the OGLE-III survey. Period-luminosity (PL) relations are derived in JHKs as well as in a reddening-free VI parameter. Within the uncertainties the BL Her…
Mixed modes seen in evolved stars carry information on their deeper layers that can place stringent constraints on their physics and on their global properties (mass, age, etc...). In this study, we present a method to identify and measure…
Using linear non-adabatic pulsation analysis, we explore the radial-mode (p-mode) stability of stars across a wide range of mass (0.2 <= M <= 50 Msun), composition (0 <= X <= 0.7, Z=0.001, 0.02), effective temperature (3 000 <= T_eff <= 40…
We compute the phase lags between the radial velocity curves and the light curves $\Delta \Phi_1= \phi^{V_r}_1 - \phi^{mag}_1$ for classical Cepheid model sequences both in the linear and the nonlinear regimes. The nonlinear phase lags…
We present the results of an extensive theoretical investigation on the pulsation behavior of Bump Cepheids. We constructed several sequences of full amplitude, nonlinear, convective models by adopting a chemical composition typical of…
Space-based photometric telescopes stirred up stellar astrophysics in the last decade, and RR Lyrae stars have not been an exception from that either. The long, quasi-continuous, high-precision data from MOST, CoRoT and Kepler revealed a…
To properly quantify possible residual systematic errors affecting the Classical Cepheid distance scale, a detailed theoretical scenario is recommended. By extending the set of nonlinear convective pulsation models published for $Z=0.02$…
We analyze Galactic, Large Magellanic Cloud and Small Magellanic Cloud Cepheids and RR Lyrae variables in terms of period-color (PC) and amplitude-color (AC) diagrams at the phases of maximum and minimum light. We compiled Galactic Cepheids…
We present time-series observations of Population \rom{2} Cepheids in the Large Magellanic Cloud at near-infrared ($JHK_s$) wavelengths. Our sample consists of 81 variables with accurate periods and optical ($VI$) magnitudes from the OGLE…
Double-mode RR Lyrae variables (i.e. radial variables which are simultaneously pulsating in both fundamental and first overtone modes) appear a fundamental tool for investigating the mass of old Population II Horizontal Branch (HB) stars.…
Cepheid masses derived from pulsations or binary dynamics are generally lower than those derived from stellar evolution models. Recent efforts have been dedicated to investigating the effects of abundances, mass loss, rotation, convection…