HOT STARS NEAR THE POSITION OF X0512-401 IN THE GLOBULAR-CLUSTER NGC-1851

We have conducted an investigation of the error box of the X-ray sourc e in NGC 1851 in the optical (ESO, La Silla) and ultraviolet (IUE) ran ges. 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 s hows clearly that NGC 1851 contains hot blue horizontal branch stars. The IUE observations detect several ''point like'' hot objects. The th ree ultraviolet detections of the northern field which contains the X- ray source are identified to correspond to two optical objects: one, c alled U1 is a normal hot blue horizontal branch star (T(eff) almost-eq ual-to 11000 K) out of the X-ray source error circle; the other one, c alled X1, is located near the centre of the X-ray error box and has ma gnitudes 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 o f 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 so urce is found to be underluminous in the UV with respect to other obse rved low mass X-ray binaries, which we interpret as due to a moderate heating of the accretion disc.