Yukawa Couplings involving Excited Twisted Sector States for ${\bf Z}_N$ and ${\bf Z}_M\times {\bf Z}_N$ Orbifolds

We study the Yukawa couplings among excited twist fields which might arise in the low-energy effective field theory obtained by compactifying the heterotic string on ${\bf Z}_N$ and ${\bf Z}_M\times {\bf Z}_N$ orbifolds.