The solid lines are Cambridge stars models of a pure helium star. Evolution proceeds from the rightmost line at large V to the left. The short-dashed lines are, from bottom to top, contours of fractional masses 0.516, 0.543 and 0.562 for n= 3. The inset is a magnification of the boxed region. The first model is at the end of the core He-burning sequence. The second model appears to have reached an SC-like limit of q≈ 0.543 according to our criterion. At this point, the star begins to expand rapidly and Teff decreases. As it evolves across the Hertzsprung gap, the gradient becomes steeper than the contours for the relevant fractional core mass.