We discuss whether with new evolutionary tracks we still have a problem fitting the Cepheids and their evolved companions on the appropriate evolutionary tracks. We find that with the Bertelli et al. tracks with convective overshoot by one pressure scale hight the problem is essentially removed though somewhat more mixing would give still better fits.

Using results of recent nonlinear hydrodynamic calculations by Morgan we find that we also have no problem matching the observed pulsation periods of the Cepheids with those expected from their new evolutionary masses, provided that Cepheids with periods less than 9 days are overtone pulsators.

We investigate possible mass loss of Cepheids from UV studies of the companion spectrum of S Mus and from the ultraviolet spectra of the long period Cepheid Carinae. For S Mus with a period of 9.6 days we derive an upper limit for the mass loss of M < 10-9 M⊙, if a standard velocity law is assumed for the wind. For Carinae with a period of 35.5 days we find a probable mass loss of M ∼ 10-5±2 M⊙.

IUE observations of δ Scuti variables were planned to study the correlations between chromospheric activity and dynamics of pulsations, convection, rotation and to search for evidence of mass loss. So far we observed the following stars: ρ Pup, β Cas, o1 Eri, K2 Boo, τ Peg, 69 Tau, 71 Tau and τ Cyg. Results and discussions on our survey may be found in Pasinetti Fracassini et al. (1990) and Fracassini et al. (1991).

Ultraviolet spectroscopic data (6 LWP and 3 SWP spectra) of 71 Tau were obtained with IUE in the year 1990, spanning an interval of 5h35rn and covering about 1.5 cycles of the pulsation period. The period, derived from new photometric observations, is 4h32m with an cimplitude of 0m.028. This variable is the most intense X-ray source in the Hyades cluster according to the results of Einstein Observatory.

Recent observations in the extreme ultraviolet have shown that the binary companion of the pulsating metallic-lined A8m: star IK Peg is a hot DA white dwarf. The EUV light curve is shown to be subject to too much photon noise to identify any periods, and simulations of the effect of a hotspot on the compact object indicate that it is unlikely that the observed photometric variability is not intrinsic to the A8m: star.

IUE observations of hot companions of Cepheids have been used to determine the temperatures of the companions. For companions on the ZAMS, the temperatures can be used to determine the luminosity of the Cepheids from the inferred absolute magnitude of the companion. The accuracy of this method is comparable to the accuracy of calibration using Cepheids in clusters. An over-tone pulsator (SU Cas) has been identified by this technique. The luminosity of the double mode pulsator Y Car agrees with that from the PLC, confirming that is a normal Population I Cepheid. The variation of the width of the instability strip (as a function of luminosity) in the HR diagram is confirmed, and overtone and double mode pulsators are shown to be near the blue edge of the instability strip. Comparison between variables and nonvariables in the HR diagram (from IUE and cluster studies) shows very little overlap. The shape of the observed instability region may be determined both by the boundary between the variables and nonvariables and also the location of the tips of the blue loops of evolutionary tracks. Finally, eight Cepheids with hot companions which are evolved beyond the ZAMS have been studied. Half the systems are matched well by evolutionary tracks with little or no core convective overshoot near the main sequence. At least two and possibly four systems, however, cannot be matched by current isochrones.

Two methods involving the observed amplitudes of radial velocity and UBVR light variations for classical Cepheids have been analysed, both being implicitly known: their principle is trivial but these methods had not yet been used systematically as indicators of duplicity.

The slope method is based on the alteration of the wavelength dependence of the light variation amplitude if either a blue or a red companion is added to the light of the Cepheid. The amplitude ratio (AR) method makes use of the fact that the companion reduces the amplitude of the light variation without observable effect on the pulsational radial velocity amplitude. This means that the ratio of these two amplitudes (Arad.vel./AB) has a larger value for binary Cepheids as compared with the single pulsators.

Each method has been applied to more than 100 Cepheids, thus allowing to study how the uncontaminated parameters (amplitude ratio and slope) depend on the pulsation period. Binary Cepheids deviate from the regular pattern in these diagrams, and a number of new binaries can be discovered in this way. The effect of duplicity is revealed by both methods independently for VZ CMa, FM Cas, CR Cep, V402 Cyg, VI154 Cyg, V440 Per and DR Vel.

AW Per is a well-known binary Cepheid. Recently Welch & Evans (1989) determined a spectroscopic orbit. Evans (1989) pointed out that the spectral type of the companion is incompatible with the mass function of the system. We re-determined the orbit of AW Per using both photometric and spectroscopic data. The result of the simultanuous least-squares fit can be seen on Fig.l. The orbital elements are very close to the results of Welch & Evans. After correcting for the companion, the Cepheid's light and velocity curves were analyzed with the surface-brightness method. The details of the analysis will be published in MNRAS (Vinko, 1992). The mass of AW Per was found to be 6 solar masses, which confirms the mass-problem of the secondary. Observations are planned to continue at Konkoly Observatory.

RR Lyrae (0.566 day period) exhibits the Blasko effect that suggests another natural mode with almost the same period as the accepted fundamental radial mode. This mode might be nonradial, but no one has done an extensive evaluation of this idea. An investigation requires a model that includes the deep composition structure where g-modes of low angular (observable) degree have weight and amplitude. An RR Lyrae model including the outer half of the mass and more than 99% of the radius, based on an asymptotic giant branch model from Hollowell (private communication), see below, was used for this study. It includes composition gradient ramps between the primordial surface hydrogen and helium and the almost pure helium shell and the one between this helium shell and the convective core that is burning helium.

Nonradial mode periods almost resonant with the radial fundamental mode period seem to occur for all low ℓ values. In addition to significant pulsation amplitudes in the composition gradient regions where the Brunt Väisälä frequency is large, these low degree and low radial order modes have near-surface amplitudes very similar to the low order radial modes. These modes are evanescent in the convective core. Classical K and γ effects give enough driving in the very low mass surface layers, so that important deep radiative damping for these modes does not completely stabilize nonradial g-mode pulsations. The g4, ℓ=1 mode gives a. double-mode RR Lyrae with Blasko effect.

A nonradial mode may not always be visible, depending on how rotation presents the nonspherical pulsations to the observer. Thus the Blasko effect might come and go, as observed for maybe 20% of all RR Lyrae variables. For many, the Blasko effect may not be observable, even when a nonradial mode is there.

The cause of the Blazhko effect, the long-term modulation of the light and radial velocity curves of some RR Lyr stars, is still not understood. The observational characteristics of the Blazhko effect are discussed in §1. Some preliminary results are presented from aomw recent of campaigns to observe RR Lyr, using the International Ultraviolet Explorer along with ground-based spectroscopy and photometry, throughout a pulsation cycle, at a variety of Blazhko phases. A set of ultraviolet light curves have been generated from low dispersion IUE spectra. In addition, the (visual) light curves from IUE's Fine Error Sensor are analyzed using the Fourier decomposition technique. The values of the parameters ϕ21 and R21 at different Blazhko phasas of RR Lyr span the range of values found for non-Blazhko variables of similar period.

The purpose: To look at period changes in pulsating variables from the point of view of stellar evolution. Is there evidence of systematic, slow changes that might be caused by the changes in mean density during passage across the Instability Strip?

The data: O – C diagrams for 67 RR Lyrae stars and Cepheids by student assistants at the Maria Mitchell Observatory, and for 88 northern Cepheids by L. Szabados

The method: Least-squares lines and parabolae (unless the O – C diagram shows that the period has changed in both directions). The rate of change of period comes from the coefficient of the square term in the parabola. The principal feature of these analyses is that the rate is taken to be non-zero only if the parabola is significantly better than the linear fit, at the 2-sigma level.

The distant globular cluster NGC7006 was one of the first clusters studied for which the distribution of stars along the horizontal branch of its C-M diagram showed evidence for a “second parameter” in addition to metallicity. Studies of the more than 60 known RR Lyrae stars in this cluster should yield some statistically significant trends or correlations which might help to identify the second parameter. In the first stage of this study (Wehlau, Nemec, Han-Ian $ Rich 1992, AJ, 103, 1583) photographic data from 1984 were combined with previously published data from the 1930's and 1950's and used to obtain period change rates for 46 variables. The median rate was found to fall one standard error below that predicted by the Yale evolutionary HB models. In addition, statistically significant evidence was found for a radial gradient in period change rates in the sense that rates for variables in the outer region of the cluster were more negative.

In the second stage of the investigation B and V magnitudes derived from CCD frames of the cluster are being used to obtain colors and to increase the number of variables for which good periods are known, in particular variables in the inner region of this centrally concentrated cluster.

We have carried out simultaneous uvbyβ photometry for several RR Lyrae stars. For each of these stars, the observed m1 index variation along the pulsation cycle is compared with that expected variation from the (Δm1*,β) grids of Rodriguez et al. (1991) for the corresponding temperatures, gravities and metallicities. The m1 index variations are also calculated using the Kurucz's models. Good agreement is found

We present BV CCD photometry of the variable star V9 in the globular cluster 47 Tuc. V, B, and (B – V) light curves are given. A colour-magnitude diagram based on four V and four B frames is given. V9's location on the diagram is considerably brighter and bluer than the edge of the red horizontal branch. Its radial velocity indicates that V9 is a member of the cluster.

Published lightcurves of 25 to 40 day classical Cepheids and of BL Herculis stars have been examined closely for evidence of alternating deep and shallow minima, as predicted by Moskalik and Buchler (1991) and Buchler and Moskalik (1992). No such effect has been detected. Two stars, SZ Mon and EN Tra, which are sometimes taken to be classical Cepheids, do show this RV Tauri-like effect, but neither star is, in fact, likely to be a classical Cepheid.

AU Peg, a short period BL Her-type variable, is unique among short period Cepheids. It is a member of a close binary system with an unseen companion. In t his paper the pulsational period variation is presented. It is shown that the period variation is very rapid and highly non-linear. It seems that there is no simple explanation of this phenomena by standard evolution or tidal interaction.

Several types of cool pulsating variables show unexplained long-term changes in brightness, typically on time scales of 10 to 20 times the basic (pulsational) period. The visual and photoelectric programs of the American Association of Variable Star Observers (AAVSO) are well-suited for detecting and studying these changes. Some examples are given here, including yellow hypergiants, RV Tauri stars, small- and large-amplitude red giant and super giant variables. The study of pulsating variables on long time scales provides “new perspectives” on their behavior.

CY Aquarii is a Population II dwarf Cepheid that has been observed on a regular basis since 1930. We present evidence from unpublished times of maximum light from Brigham Young University observations secured in the mid-1970s to the present that indicate that the period of CY Aqr has changed in a discontinuous manner. Furthermore, we find that the period has changed significantly since the epoch of the last published investigation (1988). Finally, contrary to some past claims, examination of our high time resolution light curves yields no indication of multi-periodic pulsation in CY Aqr.

A study of short period high amplitude Dwarf Cepheid stars has been undertaken in order to discriminate between the two possible models, namely Bessel's (1969) proposal of low mass Pop. II or old Pop. I. The telecsope utilized was the 1.5 m at the SPM Observatory. The Danish spectrophotometer that allows the simultaneous acquisition of data in the uvby filters and almost simultaneously in the Hβ narrow and wide niters was attached. With this advantage of simultaneous observations, no phasing adjustment was needed, eliminating the risks of losing information due to amplitude varition explained either by multiple periodicity or by the Blazhko effect. We have restricted our discussion to the descending branch and light minimum phase interval between 0.175 to 0.725. For this interval the unreddened indices (b – y)0 and (cl)0 were calculated as in Nissen (1988), which will serve to determine the effective gravity and temperature variation of both stars, through the model atmsphere calibrations by Breger (1974), based on the ATLAS and Kurucz model atmospheres. Mean log g values of 4.0 and 4.33, and mean temperatures of 7500 and 7700 K were determined for DY Her and BP Peg, respectively. It can be concluded that both pulsating stars DY Her and BP Peg are Pop.I, normal Dwarf Cepheid stars.

We have carried out simultaneous uvby photometry of the low amplitude δ Set star 28 And. Analysis of the data, using the Fourier Transform method, establishes 28 And as a monoperiodic pulsator. Using the classical O-C method, it is found that the pulsation of this star can be well described by means of a linear ephemeris with a period of P=0.d069304118 over the last twenty-four years. Amplitude variations are also shown to be present from season to season. The physical parameters of this star are determined and the nature of radial or nonradial pulsation is discussed on the basis on the derived phase shifts and amplitude ratios between Strömgren colours. The results indicate that 28 And pulsates in a nonradial mode with ℓ = 2.

ρ Pup is a δ Scuti type pulsator with relatively low amplitude (ΔV∼0.m09). Analysis of the data from different sources, using the Fourier Transform method, establishes ρ Pup as a monoperiodic pulsator. Using the classical O-C method, it is found that the pulsation of this star can be well described by means of a linear ephemeris with a period of P=0.d140881372 over more than eighty years. The nature of radial or nonradial pulsation is discussed on the basis on the derived phase shifts and amplitude ratios between different colours. The results indicate that ρ Pup is a radial pulsator.