Two low-amplitude variable supergiants in the Large Magellanic Cloud, S65-08 and S65-48 are each found to have periods of approximately 250 days. The cfptical data suggest that these stars are high-luminosity cepheid variables falling more than one magnitude brighter than any other known Cepheids in the LMC. Confirmation of the cepheid nature of these stars comes from their H-band magnitudes which place them accurately on a simple linear extrapolation of the narrower infrared Period-Luminosity relation. So it appears that the cepheid Period-Luminosity relation extends up to Mv ~ -8.5. To honour the astronomer who discovered the first of these highest-liminosity Cepheids, we have sub-classified the variables with log P > 1.8 as being “Leavitt variables”. As soon as these long-period variables are discovered in other external galaxies, reliable distances should be possible out to (m-M) ~30.

The advantages of using near-infrared photometry of Cepheids to determine distances to nearby galaxies are now well known. In this paper we summarize the current state of the infrared period-luminosity (P-L) relations for the Galaxy, the LMC, and the SMC and present composite P-L relations derived from all available photometry. We give distance moduli for the LMC and SMC based on these data and briefly report on the status of other work in progress.

The mean phase magnitudes at the IV waveband (1.05 micron) of thirteen Cepheids in the SMC and the LMC and nine Cepheids in groups and clusters in the Galaxy, are used in conjunction with periods, to construct P–L(IV) relations in these galaxies. The slopes and the dispersions of the relations are nearly the same. We derive a distance modulus of 19.11±0.07 for the SMC and 18.82±0.07 for the LMC.

As part of a long-term program to study the distance and stellar content of M33, CCD frames of a number of fields centered on known Cepheids in this galaxy (Hubble 1926, Sandage and Carlson 1983) have been obtained, in collaboration with Madore, McAlary, and Davis. The observations were made at the prime focus of the Kitt Peak 4m telescope, and reduced using the Kitt Peak RICHFLD profile fitting programs. These data are still preliminary, but quite sufficient to illustrate the power in the application of our method.

Near-infrared photometry of Cepheid variables in the Local Group galaxies, NGC 6822, IC 1613, and M33, and in the M81 Group galaxy, NGC 2403 has been used to determine new, independent distances to these objects, which are almost unaffected by dust extinction and by differences in metallicities among the galaxies.

A review of Cepheids in globular clusters finds 46 stars that are highly probable members of 21 clusters, all of intermediate to low metallicity with blue horizontal branches. In order to separate Type II Cepheids in the field from classical Cepheids, an analysis is made of the distribution of all Cepheids: 144 are sufficiently far from the galactic plane to be considered Type II. Their properties are compared with the cluster Cepheids. Some recent studies are reviewed that are improving our understanding of low-mass Cepheids.

W Virginis variables are the population II counterparts of the classical cepheids, although they do not show quite the same trends as are seen in the latter. Theoretical studies of the population II cepheids have not been very extensive until recent studies of the shorter period variables (BL Herculis variables, with periods between 1 and 10 days). The variables with periods above 10 days (up to about 50 days) have only been studied by a few authors, modelling the prototype star (W Vir) (.g. Christy 1966; Davis 1974). Although these models qualitatively reproduced the observations they were not very successful, and were based on a stellar mass (0.88 Mʘ) that now seems likely to be too high.

Light and velocity curves of some radial mode variable stars seem to indicate a resonance where the second overtone has a period exactly half that of the fundamental mode. The two classes of stars that show this resonance by bumps in their light curves are the classical Cepheids and the population II BL Her variables. We here propose that there is another resonance for the population II W Vir variables where the ratio of the first overtone to the fundamental periods is 0.5.

This is a short report on hydrodynamic modelling for BL Her and W Vir pulsating variables. We present only a qualitative summary of the results obtained, whereas detailed paper will appear soon.

We studied radial self-exciting stellar oscillations with convection ignored. The Massevitch I mixture (X=0.7, Z=0.004) was assumed for calculating the opacity coefficient. All models have a mass M=0.6Mʘ, but their absolute magnitudes are in the range from -0.5 to -3 mag. The study covers the period range from 1.3 to 20 days.

The masses of Cepheids can be obtained in several ways. If a Cepheid luminosity is known from membership in a galactic cluster, the mass-luminosity relation obtained from stellar evolution theory gives its mass. This evolution mass depends slightly on the composition, that is, the mass fraction of helium, Y, and on the the mass fraction of all the heavier elements, Z, but as we shall see later, the composition dependence is small.

The globular cluster Messier 10 has three known variables. The first two of these were discovered by one of us (Sawyer 1933) and the third by Arp (1955). Two of the variables, V2 (P=18.7226) and V3 (P=7.831), are population II cepheids while V1 appears to be an irregular variable. Another star which lies in the Schwarzschild gap on the horizontal branch is a suspected variable (Voroshilov 1971).

In this investigation, we examine the variations in the periods of the two cepheids over the interval 1912 to 1983 (for V2) and 1931 to 1983 (for V3). The study is based on photographs obtained with seven different telescopes - the Mt.Wilson 100-inch and 60-inch (1912 to 1919), the Dominion Astrophysical Observatory 72-inch, the David Dunlap 74-inch and 19-inch, the 16-inch at the University of Toronto downtown campus and the University of Toronto 24-inch at the Las Campanas Observatory of the Carnegie Institution of Washington. Some of our magnitudes have already been published (Sawyer 1938) and the remaining ones will be submitted to the Astronomical Journal for publication. We have also included material published by Arp (1955, 1957) in our study.

The University of Toronto 24-inch telescope at the Las Campanas Observatory of the Carnegie Institution of Washington has beerused for the study of variables in four southern globular clusters: NGC 6273=1119, NGC 6284, NGC 6293 and NGC 6333-M9.

The first three of these clusters lie about 2° from one another in the sky at southern declinations ranging from about 24.5° to 26.5°. They were investigated by one of us (Sawyer 1943) using photographs obtained at the Steward Observatory in Arizona in 1939. A number of variables were discovered, but even with additional David Dunlap plates, it was not possible to determine any periods because of the large southern declinations. At Las Campanas (latitude 29°S), they pass close to the zenith and therefore the periods are more readily determined. From our Las Campanas data, we have found that there are four Cepheids in NGC 6273, two in NGC 6284 and one in NGC 6293.

High precision photographic photometry indicates that two stars lying on the giant branch in the C-M diagram of M15 are small amplitude (~0.2 mag) variables. The two stars are Kustner 64 and 152. This investigation is based on plates taken with three telescopes: the Dominion Astrophysical Observatory 1.8-metre reflector, the David Dunlap 1.9-metre reflector and the Yunnan 1-metre reflector in China. The existing data is not sufficient for period determination.

There are a number of good reasons for wanting to improve our understanding of the structure and evolutionary status of horizontal-branch (HB) stars. To mention a few:

1. 1) The HB morphology of globular clusters can be used as a tool for studying the evolution of the galactic halo (Searle and Zinn 1978) and the time scale for halo collapse (Demarque 1980; Zinn 1980).

2. 2) HB stars offer an opportunity to evaluate the helium abundance of globular clusters. This can be done either by using the R-method (Cole et al. 1983) or the width of the RR Lyrae instability strip [Deupree 1977; but see also the paper by Stellingwerf (1984) in these preceedings which casts doubt on the validity of previous calculations of the dependence of the blue edge on helium content].

3. 3) HB stars may play an important role in understanding the integrated light of old stellar systems. In particular, blue HB stars seem to make a significant contribution to the ultraviolet light of elliptical galaxies. (Ciardullo and Demarque 1978; Gunn, Tinsley and Stryker 1981). At the same time, HB stars are believed to be the direct progenitors of asymptotic giant branch stars which have proved to be powerful tracers of stellar populations in nearby external systems (Blanco et al. 1980)

4. 4) The RR Lyrae variables are part of the HB population. One would wish to relate the observable properties of RR Lyrae variables to their chemical compositions and ages so as to use them: a) as distance indicators for galactic globular clusters (Sandage 1982a,b) and in the Magellanic Clouds; b) as tracers of stellar populations. This has been attempted for the galactic halo and disk by Sandage (1982a,b) and for the Magellanic Clouds by Butler et al. (1982).

UBVRI light curves are obtained for the two halo RR Lyrae variables U Caeli with period 0.420 days (73 observations) and V Caeli with period 0.571 days (42 observations). It is shown that their light curve characteristics are very similar to those of field RR Lyrae stars.

Fourier decompositions are studied for all five magnitudes and the resulting amplitude ratios and phase differences are discussed. The differences in the Fourier decomposition parameters between the five magnitudes are shown to be relatively small. Comparisons of the Fourier decomposition parameters for the two halo RR Lyrae stars with recently published data for field RR Lyrae stars show no systematic differences.

In the last few years several studies have shown that Fourier decomposition technique is a powerful method for quantitative description of light curves of pulsation variables. This technique was introduced by Simon & Lee (1981), who showed that amplitude ratios and phase differences provide a very useful description of the Hertzsprung progression for classical Cepheids. Recently, Simon & Teays (1982) discussed 70 RR Lyrae field stars.

In the present study I analyse 130 photographic mean light curves of RR Lyrae variables in ω Centauri taken from Martin (1938). I wish (i) to compare the Fourier decomposition parameters of the ω Cen RR Lyrae stars with those of the field variables as studied by Simon & Teays, (ii) to discuss the evidence for progression sequences among the ω Cen variables and (iii) to compare the basic pulsation properties of the RRab variables in ω Cen with those of classical Cepheids.

The effect of convection in RR Lyrae stars has been investigated using nonlinear models that include the effects of time dependence, turbulent pressure, convective overshooting, and convection-pulsation interaction. The convective structure reduces to that of mixing-length theory in the limit of no time dependence and thick convective zones. Initial tests suggested that convection has an important effect on the stability and pulsation mode of these stars. We now have investigated the nonlinear nature of convection at limiting amplitude. Convection serves as a limiting amplitude mechanism for these stars by causing turbulence near the phase of minimum radius that damps the driving. A comparison with Geneva observations of RR Lyrae shows good agreement in the phase dependence and amplitude of the turbulent motions.

Of twelve variables known in Messier 56 in Lyra, two, V1 and V6 are the subject of this paper. VI was discovered by Shapley (1920) and first determined by Sawyer Hogg (1942) to be a Cepheid with a period of 1.5 days. V6 was discovered by Sawyer Hogg (1940) from her early plates at the David Dunlap Observatory. Later Sawyer Hogg (1949) showed it to be an RV Tauri type with a period of 90.02 days, one of the first such to be identified in a globular cluster. A.H.Joy (1949) determined spectral type and radial velocity for both of these variables.