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tokens: b ) and Nardiello etal. ( 2015 ). To look for signatures of SP stars along M4 AGB stars in the V C UBI plane, we follow a standard procedure similar to the one outlined in Monelli etal. ( 2013 ). We first cleaned the CMD from foreground/background contamination, i.e. cluster membership has been assigned on the basis of the source position in the ( B I ) versus ( U V ) plane (see fig. 2 in Monelli etal. 2013 ). As a first step, we defined AGBs as shown in the top-left panel of Fig. 2 . Then, we plotted the selected stars in other CMDs that provide the cleanest separation of the evolved sequences to help verify the identification (see Fig. 2 ). To this aim, we cross-correlated our UBVI photometry with the 2MASS catalogue 3 to carefully inspect the position of selected AGB stars in CMDs including near-infrared colours. All selected stars are compatible with being AGBs in all diagrams displayed in Fig. 2 . This demonstrates that they cannot be RGBs migrated to the AGB sequence due to photometric errors. Indeed, no random errors can explain that the selected stars are always located in a well-defined sequence bluer of the main body of the RGB in different CMDs obtained from independent measurements/data sets. From all the above, we conclude that the selected stars are indeed AGBs. We finally note that the observed AGB/RGB ratio ( R AGB/RGB 0.44 0.23) agrees within the errors with the theoretical one ( R AGB/RGB 0.37). This by itself is obviously not a proof that all candidate AGBs are indeed AGB stars, but just a consistency check for our empirically derived value of R AGB/RGB . Figure 2. Selection of AGB star candidates in different CMDs. From top-to-bottom: V , B V ; V , B I ; and U , U I . Empty cyan symbols are AGB stars. Magenta asterisks mark candidate AGB stars in common with Marino etal. ( 2008 ). To assess whether some significant residual contamination may still be present in our AGB sample after the field-star subtraction, we used the code TRILEGAL (Girardi etal. 2005 ). We determined the fraction of Galactic field stars within a box 3030arcmin 2 (i.e. comparable to the FoV of our photometric catalogue) centred on the cluster centre that survive our cluster AGB selection. We found one star with colours and magnitudes consistent with being on the cluster AGB, to be compared with the two AGB objects rejected with our field-subtraction procedure. This implies that any residual field contamination is very likely negligible. Fig. 3 shows the V C UBI diagram, with AGB stars highlighted. As already shown by Monelli etal. ( 2013 ), the RGB shows a large C UBI spread at fixed V , that exceeds the spread due to photometric errors, and a hint of bimodality can also be observed. Figure 3. V C UBI diagram is shown. AGB stars and stars in common with the Marino etal. ( 2008 ) study are marked with the same symbols as in Fig. 2 . We display also a fiducial reference line (dashdotted line) employed to quantify the broadening of the RGB and AGB sequences