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 cd-1, which we interpret as twice the orbital frequency (19.30+/-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--31 degrees (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 kms-1 and 50 kms-1 respectively.
@article{arxiv.astro-ph/9801240,
title = {Aquila X--1: a low inclination soft X-ray transient},
author = {T. Shahbaz and J. R. Thorstensen and P. A. Charles and N. D. Sherman},
journal= {arXiv preprint arXiv:astro-ph/9801240},
year = {2009}
}