VARIABILITY OF L261 IN M 13

Abstract

After the discovery in April 2021 of the variability of the star L199 (now V63), belonging to the globular cluster Messier 13 (NGC 6205), between June and October 2021, we have studied twenty of the brightest stars, excluding the well-known red giant variables: surprisingly a good part of them shows some degree of variability in the range 0.05-0.25 magnitude in V band. In this work, we present light curves, average magnitudes, periods and amplitudes of the star L261 (2MASS J16413476+3627596), a red giant star located in the cluster, that we have studied using the CCD images obtained in the campaigns of 2019, 2020 and 2021, which we have joined the photometric measurements obtained by Osborn & Fuenmayor (1977), Kopacki et al. (2003), Deras et al. (2019) and ASAS-SN (from 2018 to 2021). The results suggest that this moderately bright star (average magnitude in 2021: 12.206 ± 0.027 V) is multiperiodic. It has at least two periods of different length which modulates its light curve over time: a period of variable duration (31 ± 5 days: between 26 and 36 days according to the campaign) joins another shorter (4.8 to 7.6 days) very marked in all the data sets used. With this discovery, Messier 13 now has 64 confirmed variable stars.