Ashoke Sen made a number of major original contributions to the subject of string theory , including his landmark paper on strong-weak coupling duality or S-duality ,  which was influential in changing the course of research in the field. He pioneered the study of unstable D-branes and made the famous Sen conjecture about open string tachyon condensation on such branes.  His description of rolling tachyons  has been influential in string cosmology . He has also co-authored many important papers on string field theory . In 1998 he won the fellowship of the Royal Society on being nominated by the theoretical physicist Stephen Hawking .  His contributions include the entropy function formalism for extremal black holes and its applications to attractors. His current research interests are centered on the attractor mechanism and the precision counting of microstates of black holes. Recently he has joined National Institute of Science Education and Research (NISER), Bhubaneswar, India as an honorary fellow.