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Context. In quiescence, the proposed optical counterpart to the neutron starlow mass X-ray binary 4U 2129+47 (V1727 Cyg) shows a spectrum consistent with alate F-type subgiant and no radial velocity variations on the 5.24 hour binaryperiod. This could imply that V1727 Cyg is a chance line of sight interloper.Radial velocity measurements, however, showed evidence for a longer term ~40km/s shift, which suggested that 4U 2129+47 could be a hierarchical triplesystem, with the F-type star in a wide orbit about the inner low mass X-raybinary. Aims. In order to confirm the long-term radial velocity shift reportedin Garcia et al. (1989) and its amplitude, we obtained spectroscopicobservations of V1727 Cyg during 1996 and 1998 with the William HerschelTelescope using the ISIS spectrograph. Methods. We determined radial velocitiesfrom the ISIS spectra by means of the cross-correlation technique with atemplate spectrum. Results. The resulting radial velocities show variationswith a maximum amplitude of ~40 km/s, confirming previous results andsupporting the F-type star as being the third body in a hierarchical triplesystem. The odds that this star could be an interloper are ~3e-6

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