We report the discovery of a peculiar horizontal branch ( HB ) in NGC 6440 and NGC 6569 , two massive and metal-rich Galactic globular clusters ( GGCs ) located in the Galactic bulge , within 4 kpc from the Galactic Center .
In both clusters , two distinct clumps are detected at the level of the cluster HB , separated by only \sim 0.1 magnitudes in the K _ { \mathrm { s } } band .
They were detected with IR photometric data collected with the “ VISTA Variables in the Vía Láctea ” ( VVV ) Survey , and confirmed in independent IR catalogs available in the literature , and HST optical photometry .
Our analysis demonstrates that these clumps are real cluster features , not a product of field contamination or interstellar reddening .
The observed split HBs could be a signature of two stellar sub-populations with different chemical composition and/or age , as recently found in Terzan 5 , but it can not be excluded that they are caused by evolutionary effects , in particular for NGC 6440 .
This interpretation , however , requires an anomalously high helium content ( Y > 0.30 ) .
Our discovery suggests that such a peculiar HB morphology could be a common feature of massive , metal-rich bulge GGCs .