On the triple nature of the X-ray source 4U2129+47 (= V1727 Cyg) New spectroscopic observations

Context. In quiescence, the proposed optical counterpart to the neutron star low-mass X-ray binary system 4U2129+47 (V1727 Cyg) shows no radial velocity variations on the binary period. This could imply that V1727 Cyg is a chance line-of-sight interloper. Radial velocity measurements, however, showed evidence of a longer term ∼40 km s−1 shift, which suggested that 4U2129+47 could be a hierarchical triple system, with a third star in a wide orbit about an inner low-mass X-ray binary. Aims. To confirm the long-term radial velocity shift reported in Garcia et al. (1989) and its amplitude, we obtained spectroscopic observations of V1727 Cyg during 1996 and 1998 with the William Herschel Telescope using the ISIS spectrograph. Methods. We determined radial velocities from the ISIS spectra by means of the cross-correlation technique with a template spectrum. Results. The resulting radial velocities show variations with a maximum amplitude of ∼ 40 km s−1, confirming previous results and supporting the observed star as being the third body in a hierarchical triple system. The odds that the star could be an interloper are ∼ 3 × 10−6. .