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This is the first of a series of papers devoted to a global study of thephotometric properties of the red stellar sequences in a complete sample of theLarge Magellanic Cloud clusters, by means of near infrared array photometry.Deep J,H,Ks photometry and accurate Color Magnitude Diagrams down to K=18.5,i.e. 1.5 mag below the red He-clump, for six intermediate age clusters (namelyNGC1987, NGC2108, NGC2190, NGC2209, NGC2231, NGC2249) are presented. Aquantitative estimate of the population ratios (by number and luminosity)between Red Giant Branch and He-clump stars for each target cluster is providedand discussed in the framework of probing the so-called Red Giant Branch phasetransition (RGB Ph-T). By using the Elson & Fall s-parameter as an ageindicator, the observed RGB population shows a sharp enhancement (both innumber and luminosity) at s=36. Obviously, the corresponding absolute agestrictly depends on the details of theoretical models adopted to calibrate thes-parameter. Curiously, the currently available calibrations of the s-parameterin term of age based on canonical (by Elson & Fall 1988) and overshooting(Girardi et al. 1995) models provide ages that well agree within 10%,suggesting that the full development of the Red Giant Branch occurs at t=700Myr and be a relatively fast event (delta t=300 Myr). However, the RGB Ph-Tepoch derived from the overshooting calibration of the s-parameter turns out tobe significantly earlier than the epoch provided by the recent evolutionarytracks by Girardi et al. (2000). A new calibration of the s-parameter based onhigh quality Color Magnitude Diagrams and updated models is urged to addressthe origin of this discrepancy and finally establish the epoch of the RGB Ph-T.Comment: ApJ, in pres

Probing the Red Giant Branch Phase Transition: Near‐Infrared Photometry of Six Intermediate‐Age Large Magellanic Cloud Clusters