Variable stars in the field of the galactic globular cluster M71

Abstract

M71 is a nearby, metal-rich globular cluster at low Galactic latitude, where field contamination and spatially variable extinction complicate colour─magnitude diagrams (CMDs) and the identification of cluster member variable stars. Our aims are (i) to construct a homogeneous census of variable stars in M71 by refining their periods and classifications and identifying new candidates, and (ii) to derive a decontaminated, differentially dereddened CMD to constrain its physical properties. We obtained Johnson─Kron─Cousins VI time-series CCD photometry and reduced it using difference image analysis. Cluster membership was established from Gaia DR3 proper motions, and a differential-reddening correction was applied across the field of view. The resulting CMD, cleaned of field stars, was compared with tailored isochrones to estimate age ( 12.9+0.9−0.8Gyr), metallicity ([Fe/H] = −0.88+0.13−0.15), mean reddening ( E(B−V) = 0.21±0.02 ), and distance modulus ( (m−M)0 = 13.01±0.06). Variable stars were identified using two complementary approaches: a periodogram-free string-length scan refined with phase dispersion minimization, and a robust inter-site screening based on median statistics combined with a generalised Lomb─Scargle significance criterion. We identified 21 variable stars not previously reported in the Catalog of Variable Stars in Globular Clusters and provided their periods, amplitudes, classifications, membership status, and light curves. This combined strategy yields a consistent picture of M71, expanding its known variable-star population and confirming parameters typical of metal-rich Galactic disc globular clusters.