On the period and peculiarity of the cool Ap star HD 203006 (theta 1 Mic).

Summary. Measurements carried out in UBV and with a multicolor medium band filter system (Maitzen, 1973) combined with uvby measurements of Morrison and Wolff (1971) yield a rotational period of 2.1219 days with epoch 2440345.32 J.D. (primary maximum in v) for the Sr-Eu-Cr star HD 203006. This is twice as large as the period derived by Morrison and Wolff (1971) and was obtained by analyzing the strong scatter in the light curves of V, Hy -wide and B when using their 1d062 period. Adopting the two day period one finds a considerable difference in height between the primary and secondary maxima. This is substantiated by relative line strength measurements of Eu 114205 and Sr ii 4215. Eu II exhibits a double wave variation with primary maximum at phase 0.5 and a moderate secondary maximum at phase 0.0, whereas Sr ii shows only a simple variation with maximum at zero phase. Using the one day period the scatter in both line variations is increased considerably. Further support for the two day period is given by the magnetic field measurements of Babcock (1958). Although there is only one numerical value in his list one notes that the sign of the magnetic field reverses an odd number of times in an odd number of days. This is compatible only with a period of an even number of days, since the theory of the Oblique Rotator requires two reversals, if any, of the magnetic polarity during one rotation. The intensity distribution with wavelength relative to the comparison star HD 203585 shows the flux to be reduced in the V band at the primary maximum of the Eu variation. A flux depression occurs at 5240 A. Both phenomena are compared with similar features in the intensity distribution of the longer period Eu star HD 125248. On the base of the magnetic accretion theory (Havnes and Conti, 1971) it is discussed whether a difference in the product of magnetic field strength and age is responsible for the different photometric behaviour of both stars. As the commonly used criteria of peculiarity, i.e. amplitudes of light and color variations, suffer from being vealed by the geometrical aspect between star and observer, it is proposed to establish a suitable photometric filter system in order to determine the degree of peculiarity from measurements of the flux depressions of cool Ap stars. Measurements of this type should be carried out in open clusters either to detect or determine the peculiarity of A type stars. Having information about the age of the cluster one should get a deeper insight into the effect of the magnetic field on the development of the peculiarity in cool Ap stars. Numerical light curves and an English version of this paper are available on request from the first author. Key words: cool peculiar A stars - rotational period - degree of peculiarity - HD 203006