The persistent X-ray source 4U 0142+614 was discovered by UHURU and soon noticed to possess an ultrasoft spectrum. In the X-ray colour-colour diagram of White & Marshall (1984) it occupies the same region of black hole candidates in their ``high state", such as LMC X-3, LMC X-1 and A 0620-00. 4U 0142+614 lies in the galactic plane (l=129 .4, b=-0 .4) and, despite its small error circle (a few arcsec), no optical or radio counterparts have yet been identified (White et al. 1987). While the X-ray luminosity of 4U 0142+614 ( (d/4 kpc) erg s ) is 1-2 orders of magnitude lower than that measured for the sources above, its spectrum (power law with energy index of 3-4) is reminiscent of high state black hole candidates.
During one of three EXOSAT observations of 4U 0142+614 carried out in
additional spectral component was detected above 3 keV within the
arcmin collimator response of the Medium Energy (ME) experiment.
Correspondingly, 25 min periodic oscillations were discovered in
the 3-10 keV energy range (White et al. 1987).
Mereghetti, Stella & De Nile (1993) pointed out that this component and
the 25 min oscillations could originate from a nearby source in
the ME field of view, namely RX J0146.9+6121, an X-ray transient
discovered with ROSAT and identified with the Be star
LSI +61 235 (Motch et al. 1991).