Isochrones representing the two possible stellar populations in Terzan 5. The basic population has Y = 0.26 and Z = 0.01 (blue full lines, isochrones of 10, 12, and 14 Gyr). The second population may either be coeval (dashed black lines, same isochrones for Y = 0.33 and Z = 0.03) or be much younger (red dash-dotted isochrones for 6 and 8 Gyr, Y = 0.29, Z = 0.03). The lines with squares represent the location for each set of isochrones at which the evolving mass has a radius equal to the Roche lobe radius of IGR J17480 in a system with orbital period P = 21.274 h.