Discovery of a pair of classical cepheids in an invisible cluster beyond the galactic bulge

We report the discovery of a pair of extremely reddened classical Cepheid variable stars located in the Galacticplane behind the bulge, using near-infrared (NIR) time-series photometry from the VISTA Variables in the VaLáctea Survey. This is the first time that such objects have ever been found in the opposite side of the Galacticplane. The Cepheids have almost identical periods, apparent brightnesses, and colors. From the NIR Leavitt law,we determine their distances with ∼1.5% precision and ∼8% accuracy. We find that they have a same totalextinction of A(V) ? 32 mag, and are located at the same heliocentric distance of ádn = 11.4 ± 0.9 kpc, and lessthan 1 pc from the true Galactic plane. Their similar periods indicate that the Cepheids are also coeval, with an ageof ∼48 ± 3Myr, according to theoretical models. They are separated by an angular distance of only 18?.3,corresponding to a projected separation of ∼1 pc. Their position coincides with the expected location of the Far 3kpc Arm behind the bulge. Such a tight pair of similar classical Cepheids indicates the presence of an underlyingyoung 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.