Exploratory electronic structure calculations have been performed with the CC2 ( simplified singles and doubles coupled-cluster) method for two conformers of the adenine ( A)- thymine ( T) base pair, with emphasis on excited-state proton-transfer reactions. The Watson-Crick conformer and the most stable ( in the gas-phase) conformer of the A-T base pair have been considered. The equilibrium geometries of the ground state and of the lowest excited electronic states have been determined with the MP2 (second-order Mlller-Plesset) and CC2 methods, respectively. Vertical and adiabatic excitation energies, oscillator strengths, and dipole moments of the excited states are reported. Of particular relevance for the photochemistry of the A-T base pair are optically dark 1 pi pi* states of charge-transfer character. Although rather high in energy at the ground-state equilibrium geometry, these states are substantially lowered in energy by the transfer of a proton, which thus neutralizes the charge separation. A remarkable difference of the energetics of the proton- transfer reaction is predicted for the two tautomers of A-T: in the Watson-Crick conformer, but not in the most stable conformer, a sequence of conical intersections connects the UV-absorbing 1 pi pi* state in a barrierless manner with the electronic ground state. These conical intersections allow a very fast deactivation of the potentially reactive excited states in the Watson-Crick conformer. The results provide evidence that the specific hydrogen-bonding pattern of the Watson-Crick conformer endows this structure with a greatly enhanced photostability. This property of the Watson-Crick conformer of A-T may have been essential for the selection of this species as carrier of genetic information in early stages of the biological evolution.