2025ApJ...985...26M

We present the analysis of stellar variability in an open cluster NGC 7209 using optical V-band CCD photometric observations. Out of 4499 stars in our study, we detected 284 candidate variables, including one identified previously. Using the machine learning algorithm HDBSCAN technique and Gaia DR3 data, we identified 246 cluster members with membership probability greater than 60%, among which 17 were identified in our study as variable stars. Based on isochrone fitting, we approximated the log(age) of the cluster to be 8.8 ± 0.2. We provide the first time-series V-band photometric study of 14 newly detected variable cluster members, detecting three potential δ Scuti, three γ Doradus, and four rotationally modulated variables, along with four possible eclipsing binaries. We detected an optical flare on 2022 November 30, from the star ID 1329 (R.A.: 331.50897; decl.: 46.44859), with an estimated emitted energy of 5.37 × 10³⁰ erg, which is below the typical energy range of a superflare; an error of a few tens of percent is attributed to the calculation of energy.