Search for X-ray emission from bona-fide and candidate brown dwarfs

Following the recent classification of the X-ray de- tected object V410 x-ray 3 with a young brown dwarf candidate (Brice ˜ no et al. 1998) and the identification of an X-ray source in Chamaeleon as young bona-fide brown dwarf (Neuh ¨ & Comer´ on 1998), we investigate all ROSAT All-Sky Survey and archived ROSAT PSPC and HRI pointed observations with bona-fide or candidate brown dwarfs in the field of view with exposure times ranging from 0.13 to 221 ks, including dedi- cated 64 ks and 42 ks deep ROSAT HRI pointed observations on the low-mass star BRI 0021 0214 and the brown dwarf Calar 3, respectively. Out of 26 bona-fide brown dwarfs, one is newly detected in X-rays, namely Oph GY 202. Also, four out of 57 brown dwarf candidates studied here are detected in X- rays, namely the young Taurus brown dwarf candidates MHO- 4, MHO-5, V410 Anon 13, and V410 x-ray 3. The M9.5-type star BRI 0021 0214 is not detected. In the appendix, we also present catalogued, but as yet unnoticed B- and R-band data for some of the objects studied here.