well as other interactions, such as string theory [@gsw1987], naturally predict the existence of scalar fields that relate to gravity. In the hyperextended inflation model [@sa1990], scalar-tensor theories of gravity play an essential role. Moreover, projects of laser interferometric gravitational wave observations [@ligo; @virgo; @geo; @tama] will be soon in practical use, so that high-accuracy tests of the scalar-tensor theories may be expected [@wz1989; @will1994; @snn1994; @sst1995; @hcnn1997].

Scalar-tensor theories are viable theories of gravity for some choices of the coupling function which is contained in the theories. Predictions of these theories in a strong field may be drastically different from those of general relativity. Recently, Damour and Esposito-Farèse [@de1993; @de1996] discovered one example of such phenomena. They showed that, for some choices of the coupling function, the configuration of a massive neutron star deviates significantly from that in general relativity, even if the post-Newtonian limit of the theory is extremely close to or even agrees with that of general relativity. This deviation in a strong field may be easily tested from binary-pulsar timing observations, if it exists, because of the extra energy loss by scalar gravitational radiation [@de1996]. The deviation from general relativity can be no longer dealt with 