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NGC 147 and NGC 185 are two of the most massive satellites of the Andromedagalaxy (M 31). Close together in the sky, of similar mass and morphologicaltype dE, they possess different amounts of interstellar gas and tidaldistortion. The question therefore is, how do their histories compare? Here wepresent the first reconstruction of the star formation histories of NGC 147 andNGC 185 using long-period variable stars. These represent the final phase ofevolution of low- and intermediate-mass stars at the asymptotic giant branch,when their luminosity is related to their birth mass. Combining near-infraredphotometry with stellar evolution models, we construct the mass function andhence the star formation history. For NGC 185 we found that the main epoch ofstar formation occurred 8.3 Gyr ago, followed by a much lower, but relativelyconstant star formation rate. In the case of NGC 147, the star formation ratepeaked only 7 Gyr ago, staying intense until ~ 3 Gyr ago, but no star formationhas occurred for at least 300 Myr. Despite their similar masses, NGC 147 hasevolved more slowly than NGC 185 initially, but more dramatically in morerecent times. This is corroborated by the strong tidal distortions of NGC 147and the presence of gas in the centre of NGC 185.

Long period variable stars in NGC 147 and NGC 185. I. Their star formation histories