Magnitude, color and spectral type of Aql X-1 in quiescence

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Direct I and V-band imaging of Aql X-1 in quiescence and during outburst maximum shows that the true optical counterpart is the interloper located 0.48" West of the previously known star. We find for the new counterpart V = 21.6 and V-I = 2.2 in quiescence, when its contribution to the total light was 12% in V and 22% in I. Analysis of this photometry and of low-resolution spectra of the sum of both stars, also taken during quiescence, shows that the likely spectral types for the previously known star and the optical counterpart are late G and late K, respectively, reddened by E(B-V) = 0.5 ±0.1.