The variable stars in the globular cluster NGC 6642

Abstract

Photometry is presented for sixteen new and two previously discovered variables within the tidal radius of NGC 6642, and for eight new and three previously known variables outside the tidal radius. Of the former, ten have well-determined periods, indicating that they are RR Lyrae stars, and an additional seven are also probable RR Lyraes; most of these seem likely to be members of the cluster. An apparent visual distance modulus for NGC 6642 of 15.6±0.4 mag is derived. This value is 0.8 mag larger than the previous estimates. The RRc stars in the cluster show period shifts appropriate to its metallicity; the RRab stars show shifts corresponding to a somewhat lower metallicity