Hecabe, named after the wife of Priam in Eurides' play of the fall of Troy.
According to Markku Savela's site, the taxon has been revised no less than 56 times, the TS being Papilio hecabe Linnnaeus 1758.
Despite its common name the Common Grass Yellow (also the Low veld Yellow,) this is appears to be the least common of our three Eurema, I have only two records, both shown here. The top one I forgot to document; the bottom two are of the same individual in our garden on an invasive balloon vine (Cardiospermum grandiflorum.)
The underside markings are similar to E. Desjardensii, but but the scalloped upper side marginal band (seen contre-jour here) and the rounded hind wing set it apart.
Steve Woodhall mentions an unnamed WSF found in Kwazulu-Natal that lacks the underside apical brown patch. There is just a hint of one in the top photo, but is entirely lacking in the bottom.
Our St John's wart (Hypericum aethiopicum) is one of its food plants.