Phase-resolved spectroscopy of the low-mass X-ray binary system 4U 1636-536/V801 Ara

4U1636-543/V801 Ara observations covering the full binary orbit of 3.8 days were obtained with the IMACS instrument on the 6.5m Walter Baade Telescope at Las Campanas. Our tomograms of the system in H-alpha and H-beta clearly detect the accretion disk but the disk is not centered on the center-of-mass of the neutron star. This offset has been seen also in the persistent NS LMXB, X1822-371 and implies disk precession. Instead of a hot spot as is expected at the point where the accretion stream hits the disk these tomograms show enhanced emission below this region. Lack of a hot spot and emission at a further point on the disk implies a gas stream interaction downstream of the hot spot as also seen in X1822-371 and other similar systems (eg EXO0748-676). The radial velocity curve of H-alpha does not show strong orbital modulation which is consistent with emission dominated by the disk. The radial velocity curves of the Bowen blend show strong modulation at the orbital period as expected for emission originating on the secondary and the tomogram suggests emission from the heated side of the secondary.