  We present an analysis of the constraints on the amplitude of primordial
non-Gaussianity of local type described by the dimensionless parameter $f_{\rm
NL}$. These constraints are set by the auto-correlation functions (ACFs) of two
large scale structure probes, the radio sources from NRAO VLA Sky Survey (NVSS)
and the quasar catalogue of Sloan Digital Sky Survey Release Six (SDSS DR6
QSOs), as well as by their cross-correlation functions (CCFs) with the cosmic
microwave background (CMB) temperature map (Integrated Sachs-Wolfe effect).
Several systematic effects that may affect the observational estimates of the
ACFs and of the CCFs are investigated and conservatively accounted for. Our
approach exploits the large-scale scale-dependence of the non-Gaussian halo
bias. The derived constraints on {$f_{\rm NL}$} coming from the NVSS CCF and
from the QSO ACF and CCF are weaker than those previously obtained from the
NVSS ACF, but still consistent with them. Finally, we obtain the constraints on
$f_{\rm NL}=53\pm25$ ($1\,\sigma$) and $f_{\rm NL}=58\pm24$ ($1\,\sigma$) from
NVSS data and SDSS DR6 QSO data, respectively.
