Graduate students facing limits on their income because of a rule restricting them to four years as teaching fellows yesterday sent letters to two Harvard administrators protesting the limit, the chairman of the Graduate School of Arts and Sciences (GSAS) Student Council, said yesterday.

The letters ask Dean Rosovsky and Peter S. McKinney, acting dean of GSAS, to set the limit by counting the number of courses taught, instead of counting the number of years spent as a teaching fellow, Panayote E. Dimitras, chairman of the council, said yesterday.

The letters also ask individual GSAS departments to give financial aid to students who remain in financial jeopardy as a result of the ruling, Dimitras said.

At a meeting with graduate students Friday, McKinney said the ruling exists because excessive teaching lengthens the time students spend completing their degree programs.