AQUILA X-1 - A LOW-INCLINATION SOFT-X-RAY TRANSIENT

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 cycles d(-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 var iations as the ellipsoidal modulation of the secondary star. We determ ine the binary inclination to be 20 degrees-30 degrees (90 per cent co nfidence) and also determine the 95 per cent upper limits to the radia l velocity semi-amplitude and rotational broadening of the secondary s tar to be 117 and 50 km s(-1), respectively.