Variability of halo carbon stars

Context. Carbon stars are among the brightest intermediate-age stars. Over one hundred have been identified in the Galactic halo. Since the halo consists essentially in an old stellar population, we believe that these C stars are trespassers and belong to streams left over by disrupted dwarf spheroidal galaxies. Aims. By performing photometric monitoring we intend to detect long-period variables among halo carbon stars. We should be in position to identify Mira, semi-regular, and irregular variables and determine their period and age group. Methods. We obtained, over several semesters, K, J, and I images centered on the C stars in order to determine their variation and periodicity. Results. We establish the period of 14 program stars and discover 13 Miras among them. Most of them belong to the 1-3 Gyr age group. Conclusions. The period distribution of the halo Miras closely matches that of the Miras of Fornax. The lack of old Miras suggests that the majority of the halo Miras likely do not originate from the Sagittarius dwarf spheroidal galaxy, which is believed to be older than 5 Gyr.