Tax element in Cost of Equity different, practice, adjusted, element, article

ba736

I often see two different formulas for CoE in practice:
CoE = Rf + MP * beta
where either MP is defined as:
1) MP = Rm - Rf
or
2) MP = Rm - Rf * (1-t)
Questions:
- What is the rationale for tax adjusting Rf in formula 2?
- Should Rf be tax adjusted when calulating MP?
- Does anyone know of an article discussing the issue?

ninja1024

Could this have been relating to a certain type of investor? Say, a tax exempt one?