  High-precision two-photon Doppler-free frequency measurements have been
performed on the CO A$^1\Pi$ - X$^1\Sigma^+$ fourth-positive system (2,0),
(3,0), and (4,0) bands. Absolute frequencies of forty-three transitions, for
rotational quantum numbers up to $J = 5$, have been determined at an accuracy
of $1.6\times10^{-3}$ cm$^{-1}$, using advanced techniques of two-color 2+1'
resonance-enhanced multi-photon ionization, Sagnac interferometry,
frequency-chirp analysis on the laser pulses, and correction for AC-Stark
shifts. The accurate transition frequencies of the CO A$^1\Pi$ - X$^1\Sigma^+$
system are of relevance for comparison with astronomical data in the search for
possible drifts of fundamental constants in the early universe. The present
accuracies in laboratory wavelengths of $\Delta\lambda/\lambda = 2 \times
10^{-8}$ may be considered exact for the purpose of such comparisons.
