The 3(1)Delta(g) and 4(1)Delta(g) Rydberg states of Na-2, which dissociate to Na(3s) + Na(4d) and Na(3s) + Na(4f), respectively, have been observed by using high-resolution cw, optical-optical double resonance spectroscopy. The potential minima of these two states are separated by 129.74 cm(-1) and they have an avoided crossing at 4.09 angstrom implying significant interaction making them a pair of twins. The results of our observation agree very well with the ab initio calculations. (c) 2007 Elsevier B.V. All rights reserved.