The variability of carbon stars in the Sagittarius dwarf spheroidal galaxy

Context. Stars have been forming in massive dwarf spheroidal galaxies continuously for several Gyr. The detection of AGB carbon stars indicates that an intermediate-age population is present. Sagittarius, the most massive, contains some 100 carbon stars. Most of them are probably variable. Aims. Using photometric monitoring we intend to detect long period variables among Sagittarius carbon stars. We should be able to identify mira variables, semi-regular and irregular variables, and to determine their period and age group. Methods. We have obtained K and J images over four semesters. These images are centered on a sample of 27 photometrically identified C stars to follow their variation and determine their periodicity. Results. We have established the period of 14 program stars and have identified 13 miras among them. From their periods we determined that most of them are in the 3−5 Gyr age group. Conclusions. The known miras, distributed over a wide area within Sagittarius dwarf spheroidal galaxy (Sgr), belong to populations of various ages and, most probably, various metallicities