We present optical photometry of XTE J2123–058 during its quiescent state taken in 1999 and 2000 .
The dominant feature of our R-band light curve is the ellipsoidal modulation of the secondary star , however , in order to fit this satisfactorily we require additional components which comprise an X-ray heated Roche-lobe filling secondary star , and an accretion disk bulge , i.e .
where the gas stream impacts the accretion disk .
The observed dip near phase 0.8 is interpreted as the eclipse of inner parts of the accretion disk by the bulge .
This scenario is highly plausible given the high binary inclination .
Our fits allow us to constrain the size of the quiescent accretion disk to lie in the range 0.26–0.56 R _ { L 1 } ( 68 percent confidence ) .
Using the distance of 9.6 kpc and the X-ray flux inferred from the heated hemisphere of the companion , we obtain an unabsorbed X-ray luminosity of 1.2 \times 10 ^ { 33 } erg s ^ { -1 } for XTE J2123–058 in quiescence .
From the observed quiescent optical/IR colors we find that the power-law index ( -1.4 ) for the spectral distribution of the accretion disk compares well with other quiescent X-ray transients .

We also re-analyse the optical light curves of the soft X-ray transient XTE J2123–058 taken during its outburst and decay in 1998 .
We use a robust method to fit the data using a refined X-ray binary model .
The model computes the light arising from a Roche-lobe filling star and flared accretion disk irradiated by X-rays , and calculates the effects of shadowing and mutual star/disk eclipses .
We obtain relatively accurate values for the binary inclination and mass ratio , which when combined with spectroscopic results obtained in paper II gives a neutron star mass in the range 1.04–1.56 M _ { \odot } ( 68 % confidence ) .