The newly proposed UU Her-type stars are discussed; their main features being long-period variability and high galactic latitude. It is unlikely that the UU Her stars originated in the plane of the galaxy as they are now located more than a kiloparsec above it. An alternative explanation of their normal Population I characteristics must be looked for.

Rho Cassiopeiae (HR 9045, HD 224014) is a bright yellow supergiant (F8pla). If it is a member of the association CAS 0B5, it has an Mv of about -9.5, and is therefore one of the most luminous yellow supergiants known (Humphreys 1978).

The brightness of ρ Cas varies in two ways: (i) it undergoes semiregular cycles with amplitudes of about and time scales of about a year, perhaps due to pulsation, and (ii) in the 1940's, it faded by more than a magnitude for 660 days (Gaposchkin 1949); as a result, it is often classified as “R CrB?”. Although various writers have pointed to a dissimilarities between p Cas and the R CrB stars, it should be noted that R CrB itself undergoes semi-regular cycles as well as fadings. Thus the scientific interest of p Cas lies in the nature of its variations, their possible relationship to each other and to the mass loss and extreme luminosity of the star.

Most of the existing data in the literature is used to search for the periodicity of five yellow supergiant variables. The pulsational mode and Q-values are discussed, being the luminosity the key parameter for their accurate determination. An attempt to find the spherical harmonic l from the light and colour variation phase shift is carried out.

HR 7308 is an apparently normal Pop. I Cepheid (Percy & Evans 1980; van Genderen 1981) with a constant period P = 1.49 days. However, its amplitude is variable with a range from about 5 to 20 km/sec in radial velocity and 0.05 to 0.30 in visual magnitude. The timescale for this variation is about 1200 days. According to Breger (1981) and Burki, et al. (1982; hereafter BMB), the star seems to be pulsating in a single modulated radial mode. Using published 0-values BMB identify this mode as a second or higher overtone. These authors also estimate a radius R = 34 ± 5 Rʘ and, based in part upon the observations of van Genderen (1981), a temperature log Te = 3.786 ± 0.01 and a gravity log g = 2.25 ± 0.25.

The short period and small amplitude of pulsation for the 1d.95 Cepheid SU Cas make it an excellent candidate for pulsation in a purely excited mode, and, as summarized by Gieren (1982), there is some evidence from recent radius determinations for this variable which suggests that it is indeed an overtone pulsator.

EU Tauri is a classical Cepheid with one of the shortest period known. A Fourier decomposition study of its V light curve by Simon S Lee (1981) revealed a peculiar position in phase-period diagrams, very similar to the star SU Cas which is probably an overtone pulsator (Gieren, 1982). This suggested the possibility that EU Tau might be another galactic overtone pulsator. In order to investigate this question, some 100 new photometric observations of this star on the UBVRI (Johnson) system were obtained with the #2 0.9m telescope of KPNO. Simultaneously, 43 CCD spectra of the star were secured on the Coudé feed telescope of KPNO which were measured for radial velocities using a correlation technique. These velocities have an internal accuracy of better than 0.5 km/s and define a complete velocity curve of EU Tau.

We have derived atmospheric abundances of the bright Cepheid a UMi in order to study the abundance anomalies in different elements. The atmospheric abundance of C, 0, Fe-peak elements Ca, Sc, Ti, Cr, Fe and heavier s-process elements Y, Ba, Ce and Sm have been derived using the method of spectrum synthesis. The abundance of carbon is derived using the C I lines in 4700A region, whereas for oxygen, the forbidden line at 6300.311A is employed. Abundances of the Fe-peak elements and s-process elements are obtained by synthesizing selected portions in the wavelength range 4330A - 4650A. The estimates of C/0 derived from the present investigation are compared with other Cepheids of similar period. The evolutionary status of a UMi is discussed in the light of these derived abundances.

A review of the phases of stellar evolution relevant to Cepheid variables of both Types I and II is presented. Type I Cepheids arise as a result of normal post-main sequence evolutionary behavior of many stars in the intermediate to massive range of stellar masses. In contrast, Type II Cepheids generally originate from low-mass stars of low metalicity which are undergoing post core helium-burning evolution. Despite great progress in the past two decades, uncertainties still remain in such areas as how to best model convective overshoot, semiconvection, stellar atmospheres, rotation, and binary evolution as well as uncertainties in important physical parameters such as the nuclear reaction rates, opacity, and mass loss rates. The potential effect of these uncertainties on stellar evolution models is discussed. Finally, comparisons between theoretical predictions and observations of Cepheid variables are presented for a number of cases. The results of these comparisons show both areas of agreement and disagreement with the latter result providing incentive for further research.

The various excitation mechanisms (eight in all) that have been proposed to account for the vibrational instability of variable stars, are surveyed. The most widely applied one is perhaps the “envelope ionization mechanism.” This can account for most of the essential characteristics of the “instability strip.” A simple explanation of the period-luminosity relation of classical Cepheids is given. A few outstanding problems in pulsation theory are also listed.

It is well known that the difference between the adiabatic pulsation periods and the corresponding non-adiabatic periods is small in classical Cepheids. However, the difference becomes significantly large in low surface-gravity models (e.g. Aikawa 1984a). In this paper we shall discuss the origins of the difference between the two pulsation periods. The weight functions for non-adiabatic pulsation periods are introduced in analogy to those of Epstein (1950) for adiabatic periods.

Finite amplitude pulsations of realistic stellar models are analyzed within the framework of a promising asymptotic perturbation approach and the results are compared with those of numerical hydrodynamic studies.

Starting with the initial understanding that pulsation in variable stars is caused by the heat engine of Hydrogen and Helium ionization in their atmospheres (A.S. Eddington in Cox 1980) it was soon realized that non-linear effects were responsible for the detailed features on their light and velocity curves. With the advent of the computer we were able to solve the coupled set of hydrodynamics and radiation diffusion equations to model these non-linear features (Christy 1968, Cox et. al. 1966). Calculations including the effects of multi-frequency radiative transfer (Davis 1975) showed that grey diffusion was adequate for modeling Cepheids but not for the RR Lyrae or W Virginis type variables.

Table 1 lists galaxies in which Cepheids are known. The early work on the detection and period determination for these stars forms the basis of subsequent studies. These later studies include the infrared photometry of Cepheids pioneered by the Toronto group and the efforts by various workers to improve the optical photometry. An example of the importance of this latter work is the recent study of M33 by Sandage (1983) in which a revised magnitude scale leads to a distance modulus 0.67 greater than that previously adopted.

The role of cepheid variables in establishing the inner distance scale to nearby galaxies is discussed. Emphasis is placed on the necessity for broad wavelength coverage in attempting to account for metalliclty differences and reddening internal to the parent galaxies. In addition linear detectors are essential in minimizing the effects of any unresolved background contibution to the photometry. Recent infrared observations of Cepheids in Local Group galaxies are surveyed and all published data on extragalactic Cepheids are presented for convenient access.

The distances of clusters containing classical Cepheids are central to the calibration of the period-luminosity relation. As a step in improving the reliability of the calibration, uvbys photometry was obtained for B stars in eight of these clusters. These data were then used to determine the distances of the clusters and, thus, the absolute magnitudes of the Cepheids and the zero point of the PLC relation (See Schmidt 1984 for details).

A new calibration of the absolute magnitudes of early-type stars in terms of the (β, c0) photometric system is used to establish the distance moduli of clusters containing Cepheids. The zero points of the period – luminosity and period – luminosity – colour relations are calculated and compared to previous determinations.

The determination of the membership of the Cepheid TW Normae to the galactic cluster Lyngå. 6 is important because TW Nor would then be the intrinsically brightest Cepheid associated with a cluster.

A series of nine photographic exposures (three each in U,B and V) were obtained during open time at the Anglo-Australian Telescope. The plates were measured with an iris photometer and the data calibrated using the photoelectric standards of van den Bergh and Harris (1976). The data for 300 stars were then analysed using a new computerized method of main sequence fitting and cluster membership determination developed by Pedreros (1984). We found 41 member stars with a mean reddening E(B-V)=1.33±0.10 and a distance modulus of 11.1±0.4 magnitudes.

Considerable effort has been spent over the years in deriving colour excesses for classical Cepheids using their observed photometric indices. Various broad band, intermediate band, and narrow band systems have been used for this purpose, all with varying degrees of success. Reddenings derived in this manner are extremely useful, and most of the factors which limit their accuracy seem to be reasonably well known. Two others, which often tend to be overlooked but which are addressed here, are the presence of photometric companions to many Cepheids and the tendency to treat interstellar reddening in a prescriptive manner. The point is not necessarily to raise doubts about the validity of published photometric reddenings for classical Cepheids, but rather to examine the possibility that one can derive slightly more reliable colour excesses using the available observational data.

As reviewed by Kholopov (1968), star counts for a variety of open clusters reveal the existence of low density coronal regions surrounding the nuclear concentrations of most star clusters. Such cluster coronae have diameters 2.5 to 5 times larger than the respective nuclear diameters for clusters which are poor to medium-rich in member stars, and have star densities only about 10% those observed in cluster nuclei. Cluster coronae therefore contain roughly 40% to 70% of the stars in an open cluster, and are subsequently a (or, more appropriately, the) major component of most star clusters.

Skeleton photoelectric sequences and plates in U, B and V have been obtained for fields surrounding all known Cepheids with P ≳ 12 days in the Southern Milky Way. The study of 14 of these fields has been completed and is discussed in this paper.