  The X$^{1}\Sigma ^{+}$ state of NaRb was studied by Fourier transform
spectroscopy. An accurate potential energy curve was derived from more than
8800 transitions in isotopomers $^{23}$Na$^{85}$Rb and $^{23}$Na$^{87}$Rb. This
potential reproduces the experimental observations within their uncertainties
of 0.003 \rcm to 0.007 \rcm. The outer classical turning point of the last
observed energy level ($v''=76$, $J''=27$) lies at $\approx 12.4$ \AA, leading
to a energy of 4.5 \rcm below the ground state asymptote.
