We present photometric and spectroscopic observations of the low mass X-ray binary GR Mus ( XB 1254 - 690 ) , and find strong evidence for the presence of a negative superhump with a period that is 2.4 \pm 0.3 % shorter than the orbital .
This provides further support that GR Mus indeed harbours a precessing accretion disk ( with a period of 6.74 \pm 0.07 day ) that has retrograde precession and is completely tilted out of the orbital plane along its line of nodes .
This tilt causes a large fraction of the gas in the accretion stream to either 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 characteristics similar to GR Mus ( i.e .
the so-called atoll sources ) .
Furthermore , we also find marginal evidence for the presence of a positive superhump , suggesting that the accretion disk in GR Mus is eccentric due to tidal resonances .
If true , than the relationship between the positive superhump period excess and the mass ratio ( q ) provides a constraint of q = M _ { donor } / M _ { NS } =0.33-0.36 .
Together with the radial velocity semi-amplitude measurements of the compact object , and previous modeling of the inclination we obtain a mass for the neutron star of 1.2 \leq M _ { NS } / M _ { \odot } \leq 1.8 ( 95 % confidence ) .