XMM-Newton 20 ks exposures of the three program stars. The red circle marks the count-extraction region at the position of each star. Left: MOS-1 image of the field of HD 19445. This subdwarf has an effective temperature and a bolometric luminosity similar to the Sun, but a much lower metallicity of ﹩{\rm{[Fe/H]}}=-2.0﹩. Center: MOS-2 image of the field of HD 25329. This metal-poor subdwarf is both cooler and less luminous than the Sun. Right: MOS-1 image of the field of HD 140283. This metal-poor subdwarf has an effective temperature similar to the Sun but a bolometric luminosity that is a factor of 4 times higher.