NTT OBSERVATIONS OF THE GLOBULAR-CLUSTER X-RAY BINARY NGC 6712-S/X 1850-086

We present new photometric and spectroscopic observations in the error box for the X-ray burster X1850-086 in NGC 6712 obtained with the New Technology Telescope at ESO (La Silla) in excellent seeing conditions. For the first time with an homogeneous set of observations, we show that the star called S by Nieto et al. (1990) and located at about 0.8 arcsec. from the X-ray/radio source position, is an UV object with B = 21.2 +/- 0.2, U-B = -0.8 +/- 0.3 and B-V = 0.2 +/- 0.5. Its dereddened colours and X-ray to optical luminosity ratio fit those of a low mass X-ray binary (LMXRB) and thus NGC 6712:S has to be considered as the optical counterpart of X1850-086. The absolute magnitude of NGC 6712:S is as faint as Mv = 5.6 +/ 0.5 and this object is thus confirmed to belong to the class of compact LMXRB underluminous in the optical. We monitored star S in the U passband during 4 h 35 and observed a dimming of the star by about 0.4 mag., at the 3 sigma level. Two 30 min long slit spectra with S in the aperture do not show any prominent spectral features characteristic of LMXRB apart from a possible filling of the Hbeta absorption line.