Long-period variables in NGC147 and NGC185

Previous studies on the stellar content of the two nearby dwarf galaxies NGC147 and NGC185 reveal a rich population of late-type giants in both systems, including a large number of carbon-rich objects. These stars are known to show pronounced photometric variability, which can be used for a more detailed characterisation of these highly evolved stars. Owing to their well-studied parameters, these Local Group members are ideal candidates for comparative studies. Trough photometric monitoring, we attempt to provide a catalogue of long-period variables (LPVs), including Mira variables, semi-regular variables, and even irregular variables in NGC 147 and NGC185. We investigate the light variations and compare the characteristics of these two LPV populations with the results found for other galaxies such as the LMC. We carried out time-series photometry in the i-band of the two target galaxies with the Nordic Optical Telescope (NOT), covering a time span of \approx2.5 years. These data were then combined with single-epoch K-band photometry, also obtained with the NOT. Narrow-band photometry data from the literature was used to distinguish between O-rich and C-rich stars. We report the detection of 513 LPVs in NGC185 and 213 LPVs in NGC147, showing i-amplitudes of up to \approx 2mag and periods ranging between 90 and 800 days. The period-luminosity diagram for each of our target galaxies exhibits a well populated sequence of fundamental mode pulsators. The resulting period-luminosity relations we obtained are compared to relations from the literature. We discuss the universality of those relations because of which, as a side result, a correction of the distance modulus of NGC185 may be necessary. Only one of our two galaxies, namely NGC185, has a significant fraction of possibly first overtone pulsators. An interpretation of this finding in terms of differences in the star-formation histories is suggested.
Comment: 10 pages (excluding online tables) and 9 figures