Evidence for a new class of extreme ultraviolet sources

Most of the sources detected in the extreme ultraviolet (EUV; 100 Ang to 600Ang) by the Rosat WFC and EUVE all-sky surveys have been identified with activelate-type stars and hot white dwarfs that are near enough to escape absorptionby interstellar gas. However, about 15% of EUV sources are as of yetunidentified with any optical counterparts. We examine whether the unidentifiedEUV sources may consist of the same population of late-type stars and whitedwarfs. We present B and R photometry of stars in the fields of seven of theunidentified EUV sources. We detect in the optical the entire main-sequence andwhite-dwarf population out to the greatest distances where they could stillavoid absorption. We use colour-magnitude diagrams to demonstrate that, in mostof the fields, none of the observed stars have the colours and magnitudes oflate-type dwarfs at distances less than 100 pc. Similarly, none are whitedwarfs within 500 pc that are hot enough to be EUV-emitters. The unidentifiedEUV sources we study are not detected in X-rays, while cataclysmic variables,X-ray binaries, and active galactic nuclei generally are. We conclude that someof the EUV sources may be a new class of nearby objects, that are either veryfaint at optical bands or which mimic the colours and magnitudes of distantlate-type stars or cool white dwarfs. One candidate for optically faint objectsis isolated old neutron stars, slowly accreting interstellar matter. Suchneutron stars are expected to be abundant in the Galaxy, and have not beenunambiguously detected.Comment: 8 pages, incl. figures, MNRAS, accepte