`Gauge Coupling Unification Constraints on String-derived Models'

An appealing proposition to explain the suppression of proton decay mediating and other operators, 
in heterotic string derived models, is the existence of a low scale Z' that forbids the undesired 
operators, and thus may arise only after Z' breaking. We will discuss the constraints on such a Z' 
arising in string derived models and focus on the constraints arising from gauge coupling unification. 
In two contrasting cases we will show that the phenomenological constraints at the electroweak scale 
necessitate that the Z' charges are embeddable in E6. We will discuss how free fermionic models with 
the required properties may be constructed.