1994A&A...284..457A

We have conducted an investigation of the error box of the X-ray source in NGC 1851 in the optical (ESO, La Silla) and ultraviolet (IUE) ranges. The central field of NGC 1851 was observed in UBV CCD photometry. Two IUE observations, with different slit orientations were used (one from archives, one obtained for this study). The optical photometry shows clearly that NGC 1851 contains hot blue horizontal branch stars. The IUE observations detect several "point like" hot objects. The three ultraviolet detections of the northern field which contains the X-ray source are identified to correspond to two optical objects: one, called U1 is a normal hot blue horizontal branch star (T_eff ∼ 11000 K) out of the X-ray source error circle; the other one, called X1, is located near the centre of the X-ray error box and has magnitudes and colours of a colder blue horizontal branch star. If X1 is interpreted as a BHB star of about 9000 K, its properties and those of U1 are sufficient to explain the IUE observations. However, we show that X1 could be the X-ray source as well. In both cases, the X-ray source is found to be underluminous in the UV with respect to other observed low mass X-ray binaries, which we interpret as due to a moderate heating of the accretion disc.