Linear adiabatic periods, period ratios and frequencies of Cepheid models with. P0 less than about 10 days have been computed, taking into account standard and nonstandard mass-luminosity relations and the new (or augmented) opacities. A comparison of the results with the observed properties of Cepheids has yielded the following conclusions:

(a)

1. A non-standard mass-luminosity relation is needed in order to have good agreement between observed stars and models on the temperature-period (or luminosity) diagram; ‘standard’ models have much lower temperatures and luminosities than observed stars.

2. (b) The linear models predict the following resonances: f2/f0 = 2 (well known) at P0 ∼ 10 days, f4/f0 = 3 at P0 ∼ 6.8 days, f4/f1 = 2 at P1 ∼ 3.2 days, and f1 + f0 = f3 at P0 ∼ 6.5 days.

3. (c) All these resonances yield effects which have been observed in light curves of Cepheids at the predicted periods: f2/f0 = 2 and f4/f0 = 3 in classical Cepheids; f4/f1 = 2 in first-overtone Cepheids; and f4/f1 = 2 and f1 + f0 = f3 in double-mode Cepheids.

The discussion will include comparison with the results obtained with nonlinear models by Moskalik et al. (1992) for fundamental-mode Cepheids, and by Aikawa (1992) for first-overtone Cepheids.

This project was initiated in 1985 by James Nemec (University of Washington) and Nicholas Suntzeff (C.T.I.O.). The goal was to study the system of ∼600 variable stars in the Sculptor dwarf galaxy. In 1987 the author became the recipient of the plate collection, which formed the basis for his Ph.D. dissertation. In this paper preliminary results are presented. Briefly, 612 stars were studied, of which 432 are van Agt (1978) stars and 180 are newly discovered variable stars. A total of 381 stars are confirmed variables. Most of these are RR Lyraes, but many anomalous Cepheids and some candidate eclipsing variables were also found. Several candidate double-mode RR Lyrae stars were also identified. The mean period of the ab-type RR Lyrae stars is 0.60±0.08 day, and the mean period of the c-type stars is 0.35±0.03 day, not unlike the mean periods of other nearby dwarf galaxies.

The Magellanic Clouds remain an ideal place to study the properties of Cepheid variables. In this paper, we review historical and current work on Cepheids in LMC and SMC clusters, present new results for NGC 1866 and NGC 2164, and describe a new technique for automated selection of Cepheid variables using two-color photometry. We also emphasize the numerous advantages of high-precision radial velocities in the study of Magellanic Cloud variables.

CCD observations have been obtained of eight Cepheids which have few recent observations, or an uncertain period, or an uncertain finder chart. All CCD images were flat-fielded, with 4-min exposures (0.5-m telescope) or 6-min exposures (0.4-m telescope). The stars chosen for observation were CY Aqr, EV Aur, V395 Cas, V588 Cas, DW Per, HZ Per, MM Per, and SX Per.

Photometry shows clean, Cepheid light curves for six of these eight stars, with the results for MM Per shown as an example below. V588 Cas did not show a periodicity in our data set (only 9 points), and the results for EV Aur will be published separately. We obtained standard deviations of comparison-star differences for all useable stars in our CCD images, to aid future observers in choosing suitable comparisons.

The parameters and output products of a cepheid survey are presented.

The Fourier decomposition has been successfully applied to several classes of pulsating variables. Antonello and Poretti (1986) and Antonello et al. (1990a) applied it to the Cepheids with P < 8 d. The latter authors redefined the s – Cepheids a Population I Cepheids that do not follow the Hertzsprung progression, but have progression of their own. The same authors proposed a new denomination (Antonell et al., 1990b): C-a stars to indicate the Classical Cepheids and C-b stars to indicate the redefined s–Cepheids.

The new photometric data obtained at La Silla and Merate Observatories (Mantegazza and Poretti, 1992) increase the evidence of a separation of Cepheids into two well defined subclasses on the basis of the Fourier parameters of their light curves.

In the ϕ21 – P plane, the s– and Classical Cepheids are characterized by two sequences well separated for P<5.5 d.

Basic features of calibrations of period ratio diagrams in terms of mass or radius can be understood from two facts: (1) The pulsation Q-parameter for the fundamental mode of Cepheid-type models increases by a factor of about 3 from periods of about 0.05 days to 100 days, whereas Q for overtones vary much less. Therefore period ratios usually decrease with increasing pulsation period in a group of variable stars or in a model sequence (calibration curve). (2) Mass over radius (M/R) is a good pulsation parameter determining Q-values and therefore also period ratios (e.g. Cogan 1970). Using the pulsation criterion with constant Q, it is easy to show that this implies that periods of models with a fixed period ratio, e.g. 0.70, to a first approximation is proportional to both M and R. Thus all calibration curves in period ratio diagrams should move towards increasing period with increasing M or R. This is confirmed in numerous cases.

Preliminary results of the period analysis of the double-mode Cepheid TU Cassiopeiae are given. Up to now 29 frequencies have been identified: the frequency of the fundamental mode pulsation and the first overtone, and their linear combinations. The amplitudes of the frequency constituents show temporal variations.

Photoelectric BV observations have been obtained with the 16-inch Vanderbilt APT over a number of recent years for five Cepheids. These data are being examined for periods, period changes, and for possible variations in the seasonal light curves.

Spectroscopic CCD observations were carried out for V1719 Cygni and the spectrum in the visual region is analysed relative to the Sun with a line-blanketed convective model atmosphere. Adopted atmospheric parameters are : an effective temperature < Teff > = 7000 K, a surface gravity logg = 3.4. Although our result is dependent on microturbulent velocity and damping constant, it was found that Mg in V1719 Cygni is nearly solar, or underabundent by 0.2 to 0.3 dex according to the analysis of 5172.684 Å MgI line which is relatively free from blending. This is inconsistent with the previous photometric result where VI719 Cygni was known as an abnormally metal rich variable. Because the analysis was given to the single magnesium line which is not a good metallicity indicator and S/N ratio was low due to poor seeing condition, the investigation for iron lines in blue region is undertaken.

A program of high resolution spectroscopic observations of Cepheids has been carried out at Mt John University Observatory for several years. Radial velocities and asymmetries have been measured for selected metallic lines using the method of Wallerstein et al (1992).

The line profiles show the largest asymmetry at phases of maximum inward velocity. The asymmetry at phases of maximum outward velocity is smaller and sometimes in the same direction as for the inward velocity maxima. Enhanced asymmetry is also noticed at phases where the bump appears on the radial velocity curve.

To date our models are unable to predict such behaviour. It is important that a satisfactory explaination for these observations be found so that the accuracy of Baade-Wesselink radius solutions for such stars can be assessed.

New high resolution spectroscopic observations of the star RR Lyrae are reported (resolution between 15,000 and 20,000). The spectra were taken in July 1990 with the 1.52m telescope of the Observatoire de Haute Provence, using the AURELIE spectrograph, and were centered around Hβ and NaD. They were distributed in time through almost an entire composite cycle (P = 13.6 hours). Photospheric FeI lines were analysed and their FWHM variations over one luminosity cycle are interpreted in the framework of the propagation of shock waves running through the low atmospheric layers.

The population I Cepheids with 1.8 < P < 7 d are a non-homogeneous group in the sense that in this period range one can find fundamental, first overtone and double-mode pulsators. This implies that these three types of stars fall approximately in the same luminosity range. The knowledge of accurate physical parameters of these stars is therefore of paramount relevance in order to detect possible significant differences among different mode pulsators.

We have made spectrophotometric observations at La Silla Observatory of six Cepheids with P < 4 d (3 first-overtone and 3 fundamental mode pulsators). From a comparison with Kurucz's models of atmospheres (1979) we have derived their Teff's during the pulsational cycle. Due to the relevance of a correct estimate of the interstellar reddening in order to obtain accurate temperatures, we have independently determined the colour excesses of the studied objects by means of observations in the uvbyβ system using the method proposed by Feltz and McNamara (1980).

Until now there have been few spectroscopic studies of cepheids in globular clusters. In this preliminary report we present sample spectra of the stars Vl and V29 in ω Cen. Eventually, we hope to use the abundance patterns, masses, and period changes of cepheids to better understand post-horizontal branch evolution in globular clusters.

New simultaneous photometric (uvbyβ) and spectrographic observations of SX Phe are described. Analysis of the light variation frequency spectrum is performed and the oscillation modes are investigated. After separating the light curves and radial velocity curve, atmospheric and physical parameters corresponding to each different oscillation mode were determined. Intrinsic (b – y), m1, and c1 values used in conjunction with a model-atmosphere grid yield a mean effective temperature < Teff > = 7590K, a mean surface gravity, < logg > = 4.02, and [Fe/H]= -0.47 for the first period. The pulsation theory and stellar model sequences yield bolometric magnitude of mass of 1.4 M⊙ and age of 2.0 Gyrs. It was found that these atmospheric parameters are almost independent upon different oscillation mode. The radial-velocity data indicates a mean radial velocity of -37km/s and a total velocity amplitude range of 38km/s.

Atmospheric parameters and abundances of 25 elements were determined from two spectrograms of TU Cas (with dispersion 9 Å/mm) obtained in 1977 and 1990. We find Teff= 5860 - 6000K; logg = 1.0 - 1.5; and vt = 2.9 - 5.3 km/s.

The value of [Fe/H] ∼ -0.5 testifies to the fact that TU Cas is deficient in metals compared with other double-mode Cepheids. More exact values of [Fe/H] for 9 double-mode Cepheids in the Southern Hemisphere (Barrell 1982), V367 Set (a member of the open cluster NGC 6649) and our value for TU Cas all show a good correlation with P1/P0. As the atmospheric Fe abundance increases, the ratio P1/P0 decreases.

Our measured values for abundances (in brackets) of various elements in the atmosphere of TU Cas are: C (8.26), Na (6.32), Mg (7.07), Si (6.85), S (6.96), Ca (5.88), Sc (2.71), Ti (4.34), V (3.35), Cr (5.29), Mn (4.92), Fe (6.94), Co (4.47), Ni (5.70), Zn (3.63), Sr (2.00), Y (2.17), Zr (2.21), Ba (1.41), La (1.08), Ce (0.96), Nd (0.82), Sm (0.42), Eu (1.04), and Gd (0.92).

A programme to measure precise radial velocities of late-type stars is being carried out at the 1.22-m telescope of the Dominion Astrophysical Observatory (DAO). Wavelength-calibration fiducials are imposed directly on the stellar spectra by passing the starlight through a controlled hydrogen fluoride (HF) absorption cell placed in front of the coude spectrograph. Presently, the primary targets of the programme are bright G, K, and M giants. Preliminary results confirm the low-amplitude, radial-velocity (RV) variability of the yellow giants discovered at the Canada-Prance-Hawaii telescope using the HF technique. These yellow giants and additional bright candidates are now being continually monitored at DAO. Preliminary results also indicate that the “yellow giant” variability extends to the early-M giants. In addition to the RV variations, the data also yield information on the simultaneous variability of the Ca II λ8662 line, Teff, as well as the R - I index of the stars.

The hydrogen fluoride (HF) absorption cell technique has been used at the Canada-France-Hawaii Telescope for over a decade to monitor the radial velocity variability of nearby, solar-type stars in a search for substellar companions. As a complement to this program, we have also been monitoring a select group of G and K subgiants, giants and supergiants, which have all proved variable. We present here a brief summary of our analysis.

Several photometric studies conducted in the 1970's indicate that at least half the stars in the Cepheid instability strip are stable at the level of 0.02 mag (Fernie & Hube 1971; Percy 1975; Fernie 1976; Percy, Baskerville, & Trevorrow 1979). A precision radial velocity survey of these “stable stars” is currently being conducted by the author. Radial velocity errors have been reduced to a few tens of meters per second with the use of an iodine absorption cell (Marcy & Butler 1992).

Extremely low amplitude (200 m/s) periodic radial velocity variations have been found for HR 7796, an F8Ib supergiant (Butler 1992). Two and a half complete cycles have been monitored over three observing runs. The period is found to be 11.87 days. Observations of the reference star HR 509 (G8 V) taken on the same nights, show no periodicities and a scatter of just 20 m/s. Both the period and the shape of the velocity curve appear similar to a normal F8Ib Cepheid. If HR 7796 is a Cepheid, it is the smallest amplitude Cepheid by more than an order of magnitude.

New radial velocities by Butler are used to recalculate the absolute magnitudes of Eta Aquilae and X Cygni using the Barnes-Evans method. The new velocities have little effect on the result for Eta Aquilae, but the absolute magnitude of X Cygni is found to be 0.40 mag fainter than that obtained earlier.