A time-series VI study of the variable stars of the globular cluster NGC 6397

Understand

We present a new time-series VI CCD photometry of the globular cluster NGC 6397, from which we obtained and analysed the light curves of 35 variables carefully identified in the cluster field.

• We assessed the membership of the variables with an astrometric analysis based on \emph{Gaia} DR2 data.
• The cluster colour-magnitude diagram was differentially dereddened and cleaned of non members, which allowed us to fit isochrones for [Fe/H]$ = -2.0$ dex in the range 13.0--13.5 Gyr, for a mean reddening $E(B-V)=0.19$, and a distance of 2.5 kpc.
• This distance was confirmed using the period-luminosity relation for the cluster's five SX Phoenicis variables (V10, V11, V15, V21, and V23) present among its blue stragglers, yielding $2.24\pm0.13$ kpc.