Syed Zain, S. F., N. Ahmad, and M. Z. Zainuddin. “Helium λ 4009.27 Ã… Phase Variation in Alpha Virginis”. Malaysian Journal of Science, vol. 26, no. 2, Aug. 2007, pp. 143-9, http://mjs.um.edu.my/index.php/MJS/article/view/8082.