D. Bailin et al., YUKAWA COUPLINGS INVOLVING EXCITED TWISTED SECTOR STATES FOR ZETA(M) X ZETA(N) ORBIFOLDS, Physics letters. Section B, 311(1-4), 1993, pp. 110-116
A study is made for Z(M) x Z(N) orbifolds of the modification of the f
orm of the twisted sector Yukawa couplings when some of the states inv
olved are excited twisted sectors rather than twisted sector ground st
ates.