OPEN EUROPEAN JOURNAL ON VARIABLE STARS Januray 2024 http://oejv.physics.muni.cz ISSN 1801-5964 DOI: 10.5817/OEJV2024-0244 1 STUDY OF THE VARIABLE STARS V10 AND C6 IN MESSIER 13 VIOLAT-BORDONAU, FRANCISCO Observatorio Astronómico Norba Caesarina, 10195, Cáceres, Spain, fviolat@gmail.com Abstract: We have studied the variable star V10 and the new variable candidate C6 (announced in 2019), located in Messier 13, using the filtered CCD images -in the V band- obtained in the campaigns of 2019, 2020 and 2021. We have previously determined their precise  and  coordinates (J2000) to identify them in a crowded field. We find the V10 constant in light in the three campaigns: the star is listed in the Catalogue of Variable Stars in Globular Clusters as non-variable. C6 has presented low-level variations from 0.102 to 0.138 mag without regularity; these values are lower than 0.44 mag in the V band detected by its discoverers. Although we find different periods, none draw a coherent light curve. We have also analysed the photometric measurements obtained by ASAS-SN between 2019 and 2023 in the SLOAN-g band, identifying oscillations with a maximum amplitude equal to 0.188 mag;. However, we find two possible periods, none producing a quality light curve. All data suggest that C6 is a new variable star of type L, irregular because no single period fits all of the data: we propose the name V65 for it. 1 Introduction This paper aims to investigate one (C6) of the 15 candidate variable stars in Messier 13, presented in 2019, to confirm their variable status using our more extensive photometric data. We also take this opportunity to publish some corrections of mistakes in the literature regarding variable star identification and astrometric coordinates of V10 and C6. Recently, Deras et al. (2019, hereafter DER19) presented fifteen candidates for variables in Messier 13, nominating them C1 to C15, with amplitudes ranging from 0.04 (C1 and C13) to 0.44 mag (C6) in the V-band. They provided  and  coordinates (J2000) for these candidates, along with a precise chart of the cluster's variables and candidates (Figure 2 in their work). After verifying the position of C6, we were noted to coincide with the variable star V10 (Sawyer, 1940), also known as L487 (Ludendorff, 1905), with a slight difference. The electronic version of the Catalogue of Variable Stars in Globular Clusters (Clement et al., 2022) indicates that its position is very close to that provided by Sawyer; both stars could be the same misidentified star. V10 is located not far from the core (Figure 1), in its southern part, between the variable stars V24 (to the east) and V11 (to the west). In this position, a chain of three stars appears, named, in order of right ascension, L470 = BARN 103 (Barnard, 1931), BARN 105, and L487 (V10). Taking BARN 105 as a reference, both stars are at distances of 2.80” (V10) and 2.33” (L470) according to SIMBAD; however , if we use V10 as a reference, the distance to L470 is 4.55”, and to C6 is 5.02”. A careful examination of our best images (Figure 2) shows only one star in the position of L470, so both stars (L470 and C6) are the same, with a small