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We present photometric and spectroscopic observations of the low mass X-raybinary GR Mus (XB 1254-690), and find strong evidence for the presence of anegative superhump with a period that is 2.4+/-0.3% shorter than the orbital.This provides further support that GR Mus indeed harbours a precessingaccretion disk (with a period of 6.74+/-0.07 day) that has retrogradeprecession and is completely tilted out of the orbital plane along its line ofnodes. This tilt causes a large fraction of the gas in the accretion stream toeither over- or underflow the accretion disk instead of hitting the disk rim,and could be a feature of all low mass X-ray binaries with characteristicssimilar to GR Mus (i.e. the so-called atoll sources). Furthermore, we also findmarginal evidence for the presence of a positive superhump, suggesting that theaccretion disk in GR Mus is eccentric due to tidal resonances. If true, thanthe relationship between the positive superhump period excess and the massratio (q) provides a constraint of q=M_donor/M_NS=0.33-0.36. Together with theradial velocity semi-amplitude measurements of the compact object, and previousmodeling of the inclination we obtain a mass for the neutron star of1.2<M_NS/M_sun<1.8 (95% confidence).

The origin of the tilted disk in the low mass X-ray binary GR Mus (XB 1254-690)