Interpolated ZAHB loci (red lines) from the original theoretical set of ZAHB loci (blue lines). Open triangles show the locus where the convective envelope dissapears. Upper panel: Z interpolation (Z increases from top to bottom) with constant Y = 0.245 and M_i = 0.9 M_⊙. Middle panel: Y interpolation (Y increases from bottom to top at the red ZAHB and inversely at the blue ZAHB) with constant Z = 0.0016 and M_i = 0.9 M_⊙. Bottom panel: M_i (M_i increases from bottom to top at the blue ZAHB) with two constant Z (Z = 0.0016 and Z = 0.00016) and Y = 0.245. Dots are the initial points for each ZAHB locus, with the mass increasing from bottom to top.