[en] The radial velocity of Pleione shows oscillations with a range of 10 km/sec and a period of about four months. The mean velocity is +5.5 km/sec. The spectrum of the shell, first discovered by McLaughlin and Mohler in 1938, has gradually become stronger and now resembles the metallic spectrum of α Cygni. Dilution effects are conspicuous in the weakness of Mg II and Si II. Among the lines of ions having metastable lower levels, Ni II and Fe II became conspicuous in 1940, Ti II in 1941, and Mn II in 1942. This order of development is not consistent with the ordinary theory of ionization, and its explanation must be sought in the conditions of excitation of the metastable levels in the shell. The central intensities of the cores of the H lines are about 10 per cent—roughly one-half or one-third of the central intensities of the corresponding lines in α Cygni. This is explained as a consequence of the reduced re-emission which is thrown back within the shell into the emerging beam of radiation from the star. The metallic lines on the violet side of the Balmer limit and between the higher members of the series are relatively much stronger than in α Cygni. This is due to the semitransparency of the shell, on the one hand, and to the absence of Stark effect wings in the shell, on the other.
