  The low mass X-ray binary Aquila X-1 is one of the most active neutron star
X-ray transients. Despite it has a relatively bright quiescent optical
counterpart, the detection of its companion has been hampered by the presence
of a nearby interloper star. Using the infrared integral field spectrograph
SINFONI on the VLT-8.2m telescope, we unambiguously single out Aquila X-1 from
the interloper. Phase-resolved near infrared spectroscopy reveals absorption
features from a K4 +- 2 companion star moving at a projected velocity of K_2=
136 +- 4 km/s. We here present the first dynamical solution and associated
fundamental parameters of Aquila X-1, imposing new constraints to the orbital
inclination (36 deg < i < 47 deg) and the distance (d = 6 +- 2 kpc) to this
prototypical neutron star transient.
