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We report the discovery of a pair of extremely reddened classical Cepheidvariable stars located in the Galactic plane behind the bulge, usingnear-infrared time-series photometry from the VVV Survey. This is the firsttime that such objects have ever been found in the opposite side of theGalactic plane. The Cepheids have almost identical periods, apparentbrightnesses and colors. From the near-infrared Leavitt law, we determine theirdistances with ~1.5% precision and ~8% accuracy. We find that they have a sametotal extinction of A(V)~32 mag, and are located at the same heliocentricdistance of <d>=11.4+/-0.9 kpc, and less than 1 pc from the true Galacticplane. Their similar periods indicate that the Cepheids are also coeval, withan age of ~48+/-3 Myr, according to theoretical models. They are separated byan angular distance of only 18.3", corresponding to a projected separation of~1 pc. Their position coincides with the expected location of the Far 3 kpc Armbehind the bulge. Such a tight pair of similar classical Cepheids indicates thepresence of an underlying young open cluster, that is both hidden behind heavyextinction and disguised by the dense stellar field of the bulge. All ourattempts to directly detect this "invisible cluster" have failed, and deeperobservations are needed.

Discovery of a Pair of Classical Cepheids in an Invisible Cluster Beyond the Galactic Bulge