086215

Very recently comma it has been discovered that ZTF J213056.71+442046.5 is a binary system composed of a white dwarf orbiting a hot subdwarf star in only 39 min. This is the shortest orbital period of any sdOB+WD binary. The orbital separation implies that the subdwarf fills its Roche-lobe comma making this system an optimal candidate for observations at X-ray energies. The likely presence of X-ray emission is further strengthened by the finding that the periodic modulation of the optical light curve can be explained only invoking the presence of an accretion disk around the white dwarf comma which occults the subdwarf mass donor star. X-ray observations can give important information on the mass transfer rate comma on the properties of the WD and on the evolution of this unique binary.