V553 Centauri: a type II cepheid with an overabundance of carbon.

The Cepheid V553 Centauri (P = 2d.o6) has strong C2, CH and CN bands in its spectrum. These show a pronounced variation with phase, being strongest at minimum light. Their presence implies a considerable overabundance of carbon in the star's atmosphere. The radial velocity curve shows a phase lag similar to that of other Cepheids. The radial velocity is -6 km 1 The light and colour curves show the characteristic features of a globular cluster Cepheid of this period. RT TrA (P = 1d.95), for which fragmentary data are available, is very similar. V553 Cen and RT TrA probably belong to the disk rather than the halo population. Theory predicts an excess of carbon in the atmospheres of some of the longer period Type II Cepheids. The possibility that V553 and RT TrA are related to the Cepheids found in dwarf spheroidal galaxies, which are confined to short periods, is discussed.