We present the analysis of simultaneous X-ray ( RXTE ) and radio ( VLA ) observations of two atoll-type neutron star X-ray binaries : 4UÂ 1820–30 and Ser X-1 .
Both sources were steadily in the soft ( banana ) X-ray state during the observations .
We have detected the radio counterpart of 4UÂ 1820–30 at 4.86Â GHz and 8.46Â GHz at a flux density of \sim 0.1 Â mJy .
This radio source is positionally coincident with the radio pulsar PSRÂ 1820–30A .
However , the pulsar ’ s radio emission falls rapidly with frequency ( \propto \nu ^ { -3 } ) and we argue that the X-ray binary ’ s radio emission is dominant above \sim 2 Â GHz .
Supporting this interpretation , comparison with previous observations reveals variability at the higher radio frequencies that is likely to be due to the X-ray binary .
We have detected for the first time the radio counterpart of Ser X-1 at 8.46 GHz , also at a flux density of \sim 0.1  mJy .
The position of the radio counterpart has allowed us to unambiguously identify its optical counterpart .
We briefly discuss similarities and differences between the disc-jet coupling in neutron star and black hole X-ray binaries .
In particular , we draw attention to the fact that , contrary to other states , neutron star X-ray binaries seem to be more radio loud than persistent black hole candidates when the emission is ‘ quenched ’ in the soft state .