  Testing gravitational theories by binary pulsars nowadays becomes a key
issue. For the general screened modified gravity (SMG), the post-Keplerian
parameters in the neutron star (NS) - white dwarf (WD) binaries differ from
those of general relativity (GR), and the differences are quantified by the
scalar charge $\epsilon_{\scriptscriptstyle\rm WD}$ of WD. After deriving the
constraints on $\epsilon_{\scriptscriptstyle\rm WD}$ from four different NS-WD
binaries, we find that $\epsilon_{\scriptscriptstyle\rm WD}$ is different from
zero at the 2$\sigma$ level in all the cases studied, and there exists an
inverse correlation between masses and scalar charges of low-mass WDs, which is
consistent with the screening mechanisms. In particular, two independent
binaries with measured radii of WDs follow the coincident constraints on the
vacuum expectation value of the scalar field. These self-consistent results
indicate that the observations in NS-WD binary pulsars seem in favor of SMG,
rather than GR.
