Spectroscopic observations of the quiescent neutron star system 4 U 2129 + 47 ( = V 1727 Cyg ) ( Research Note )

Context. In quiescence, the proposed optical counterpart to the neutron star low mass X-ray binary 4U 2129+47 (V1727 Cyg) shows a spectrum consistent with a late F-type subgiant and no radial velocity variations on the 5.24 hour binary period. This could imply that V1727 Cyg is a chance line of sight interloper. Radial velocity measurements, however, showed evidence for a longer term ∼40 km s−1 shift, which suggested that 4U 2129+47 could be a hierarchical triple system, with the F-type star in a wide orbit about the inner low mass X-ray binary. Aims. In order 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 F-type star as being the third body in a hierarchical triple system. The odds that this star could be an interloper are ∼ 3 × 10−6.