- These are the classical Cepheids, or S Cephei variables. Comparatively young objects that have left the main sequence and evolved into the instability strip of the HR diagram, they obey the well-known Cepheid period-luminosity relation and belong to the young disk population. DCEP stars are present in open clusters. They display a certain relation between the shapes of their light curves and their periods. DCEPS (subtype) -These are S Cep variables having light amplitudes < 0".'5 in V (< 0".'7 in B) and almost symmetrical light curves (M-m approx. 0.4-0.5 periods); as a rule, their periods do not exceed 7 days. They are probably first-overtane pulsators and/or are in the first transition across the instability strip after leaving the main sequence.

Traditionally, both 8 Cep and W Vir stars are quite often called Cepheids because it is often impossible to discriminate between them on the basis of the light curves for periods in the range 3d-I0 d. However, these are distinct groups of entirely different objects in different evolutionary stages. One of the significant spectral differences between W Vir stars and Cepheids is








A 70


0.25 0.50 0.75 1.00

Figure 4.6. light 1

curve of the DCEP-typ, variable star, S Cep Cycle phase is indicated along the horizontal axis. Data I provided by the HIPPARCOS mission. (j,.

with permission.

the presence, during a certain phase interval, of hydrogen-line emission in the former and of Ca 11 H and K emission in the latter. GCVS

As indicated within the GCVS description, 5 Cepheids and W Vir stars are difficult to discriminate between; in many cases, perhaps impossible without spectral analysis. As a result, you must take care when attempting to distinguish between these two types of similar variable stars (Figure 4.6).

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