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High-precision two-photon Doppler-free frequency measurements have beenperformed 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, forrotational quantum numbers up to $J = 5$, have been determined at an accuracyof $1.6\times10^{-3}$ 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-Starkshifts. 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 forpossible drifts of fundamental constants in the early universe. The presentaccuracies in laboratory wavelengths of $\Delta\lambda/\lambda = 2 \times10^{-8}$ may be considered exact for the purpose of such comparisons.

High-precision laser spectroscopy of the CO A$^1\Pi$ - X$^1\Sigma^+$ (2,0), (3,0) and (4,0) bands