We present a search for long-period variable ( LPV ) stars among giant branch stars in M15 which , at [ Fe/H ] \sim –2.3 , is one of the most metal-poor Galactic globular clusters .
We use multi-colour optical photometry from the 0.6-m Keele Thornton and 2-m Liverpool Telescopes .
Variability of \delta V \sim 0.15 mag is detected in K757 and K825 over unusually-long timescales of nearly a year , making them the most metal-poor LPVs found in a Galactic globular cluster .
K825 is placed on the long secondary period sequence , identified for metal-rich LPVs , though no primary period is detectable .
We discuss this variability in the context of dust production and stellar evolution at low metallicity , using additional spectra from the 6.5-m Magellan ( Las Campanas ) telescope .
A lack of dust production , despite the presence of gaseous mass loss raises questions about the production of dust and the intra-cluster medium of this cluster .