2011A&A...532A..78L

Previous studies on the stellar content of the two nearby dwarf galaxies object NGC147 and object 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. Through photometric monitoring, we attempt to provide a catalogue of long-period variables (LPVs), including Mira variables, semi-regular variables, and even irregular variables in object NGC147 and object 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 ≃2.5 years. More than 30 epochs were available for a period search. 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 object NGC147 , showing amplitudes Δi of up to ≃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 object NGC185 may be necessary. A value of (m-M)=24.30mag seems to be more appropriate to match the observed data. Only one of our two galaxies, namely object 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.