Astron. Astrophys. 341, 480-486 (1999)

1. Introduction

The first paper of this series (Dziembowski & Jerzykiewicz 1996, henceforth Paper I) was devoted to the Cephei star 16 (EN) Lacertae. Presently we turn to 12 (DD) Lac = HR 8640, one of the longest known variables of this type. The star was discovered to have variable radial-velocity by Adams (1912). The main period of the variation, equal to 0 @ was derived by Young (1915). This value was confirmed by all subsequent investigators. Young (1915) has also noticed that the amplitude and the phase of maximum of the velocity curve vary from cycle to cycle.

The variability of brightness of 12 (DD) Lac was discovered photoelectrically by Stebbins (1917) and Guthnick (1919). The photometric period turned out to be equal to Young's spectrographic one. The light-curve also showed the cycle-to-cycle variations.

Following this early work, 12 (DD) Lac was the subject of numerous investigations, including the classic work of Struve (1951) and an international observing campaign organized in 1956 by de Jager (Abrami et al. 1957, de Jager 1963). A review of observations of 12 (DD) Lac throughout 1977 has been given by Jerzykiewicz (1978). The most recent references are Mathias et al. (1994), Pigulski (1994), and Aerts (1996).

All modern analyses agree that the above-mentioned cycle-to-cycle variations of the star's radial-velocity and light curves result from interference of six harmonic terms. The frequencies of five of them are confined to a narrow interval from about 4.2 to 5.5 d^-1. The amplitudes of these five terms, derived by Jerzykiewicz (1978) from the 1956 V-magnitude observations of Abrami et al. (1957), are plotted in Fig. 1 as a function of frequency. The terms are numbered in the order of decreasing amplitude. The frequency of the fifth term, not shown in the figure, is equal to the sum of the first and the fourth, . The frequencies , , and form an equidistant triplet.

Fig. 1. Frequency spectrum of 12 (DD) Lac: the amplitudes of five harmonic terms are plotted as a function of frequency. The combination term, , is not shown

In Sect. 2 we present the data: the star's position in the plane, the frequencies, and the available determinations of the spherical harmonic degrees, , and orders, m, of the observed modes. Details of deriving can be found in the Appendix. In Sect. 3 we consider three possibilities of accounting for the equidistant triplet. As in Paper I, we limit the analysis to . Sect. 4 contains the summary.

© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998
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