We report new non-linear pulsation models of the helium stars V652 Her and BX Cir. Linear theory has previously shown their pulsations to be due to iron-group bump instability. Recent high-resolution spectroscopic observations have provided high-precision measurements of their radial velocity curves and of their radii. Their masses remain less well determined. A hydrodynamic code including recent OPAL opacity data has been used to construct the models. These are compared with the observational data. In particular, we attempt to reproduce accurately the observed radial velocity and luminosity curves. The results impose additional constraints on those stellar dimensions, including mass, which remain poorly determined by observation. Final results show a model for V652 Her which reproduce the observed velocity and luminosity curves with 0.7 Msun, and 23 400 K. For BX Cir, the mass must lie between 0.50 and 0.38 Msun, if the temperature is in the range 22 400-24 000 K. However, the luminosity of the models is smaller than that measured directly by a factor of two.