We present optical photometry of the X-ray transient XTE J2123-058, obtained in 1998 July-October. The light curves are strongly modulated on the 5.95-h orbital period, and exhibit dramatic changes in amplitude and form during the decline. We used synthetic models, which include the effect of partial eclipses and X-ray heating effects, to estimate the system parameters, and we constrain the binary inclination to be i=73°+/-4. The model is successful in reproducing the light curves at different stages of the decay by requiring the accretion disc to become smaller and thinner by 30per cent as the system fades by 1.7mag in the optical. From August 26 the system reaches quiescence with a mean magnitude of R=21.7+/-0.1 and our data are consistent with the optical variability being dominated by the ellipsoidal modulation of the companion.