We report the discovery of a pair of extremely reddened classical Cepheid variable stars located in the Galactic plane behind the bulge , using near-infrared time-series photometry from the VVV Survey .
This is the first time that such objects have ever been found in the opposite side of the Galactic plane .
The Cepheids have almost identical periods , apparent brightnesses and colors .
From the near-infrared Leavitt law , we determine their distances with \sim 1.5 \% precision and \sim 8 \% accuracy .
We find that they have a same total extinction of A ( V ) \simeq 32 mag , and are located at the same heliocentric distance of \langle d \rangle = 11.4 \pm 0.9 kpc , and less than 1 pc from the true Galactic plane .
Their similar periods indicate that the Cepheids are also coeval , with an age of \sim 48 \pm 3 Myr , according to theoretical models .
They are separated by an angular distance of only 18.3 ^ { \prime \prime } , corresponding to a projected separation of \sim 1 pc .
Their position coincides with the expected location of the Far 3 kpc Arm behind the bulge .
Such a tight pair of similar classical Cepheids indicates the presence of an underlying young open cluster , that is both hidden behind heavy extinction and disguised by the dense stellar field of the bulge .
All our attempts to directly detect this “ invisible cluster ” have failed , and deeper observations are needed .