We have obtained I -band photometry of the neutron star X-ray transient Aql X–1 during quiescence .
We find a periodicity at 2.487 c d ^ { -1 } , which we interpret as twice the orbital frequency ( 19.30 \pm 0.05 h ) .
Folding the data on the orbital period , we model the light curve variations as the ellipsoidal modulation of the secondary star .
We determine the binary inclination to be 20 ^ { \circ } –31 ^ { \circ } ( 90 per cent confidence ) and also 95 per cent upper limits to the radial velocity semi-amplitude and rotational broadening of the secondary star to be 117 km s ^ { -1 } and 50 km s ^ { -1 } respectively .