A new photometric study of M14 (NGC 6402): an interpretation of the horizontal branch and beyond

We present a CCD $VI$ photometric study of the globular cluster M14. Particular attention is given to variable stars. This allowed new classifications and membership considerations. New variables are reported; 3 RRc, 18 SR 1 SX Phe. The Fourier decomposition RR Lyrae light curves lead mean metallicity [Fe/H]$_{\rm ZW}= -1.3 \pm 0.2$. Several independent methods yield distance $9.36 0.16$ kpc. A Colour-Magnitude diagram outlined by members enabled matching with theoretical predictions isochrones zero-age horizontal branches, whose fitting observations in good agreement above metallicity. Oosterhoff type M14 confirmed as Oo-int, pulsating mode distribution stars on HB shows that bimodal region instability strip shared RRab RRc By modelling mass loss at RGB after He flash events, we were able represent blue tail HB, using core 0.48 $M_{\odot}$ total masses 0.52-0.55 $M_{\odot}$. progenitor star MS 0.84 reaches about 12.5 Gyrs, consonant previous age determinations cluster. Type II Cepheids may be interpreted products post-HB evolution, driven complex processes involving burning very thin low hydrogen helium shells these their minuscule envelopes. No evidences found favor being extragalactic origin.