Effects of temperature, phase offset, and gas motion on the behavior of R for r_gas = 2r _⋆  for six helium-like ions. The ions are C v (top left), N vi (middle left), O vii (bottom left), Ne ix (top right), Mg xi (middle right) and Si xiii (bottom right). Simulation temperatures are 10 000, 20 000, 30 000 and 40 000 K (red, green, blue, and purple respectively). “GF” refers to simulations where the gas remains at a fixed distance from the center of the star, while “GM” signifies simulations where the gas moves in time and proportion to the radial motion of the star. Thus the solid line is at fixed distance with no pulsation radial-temperature phase offset; the dotted line is moving gas with no offset; the dashed line is fixed gas with an offset; and the dot-dashed line is moving gas with a phase offset. Note that the scale of the plots for the lighter elements is reduced to show the range of variation, which can be quite small.