Interacting binary DB white dwarf stars.

A group of four close, semidetached, ultra-short period binary systems, is discussed. In each of these systems, both the accreting primary star and the donating secondary star, are He white dwarfs (DBWDs). The main particularity of these objects is a remarkable absence of H in their spectra. On the basis of IUE and X-ray (mainly EXOSAT) observations of these four interacting binaries, some preliminary results are presented. Previous published reports on UV spectroscopy and X-rays detections, are also briefly reviewed.