This is called underload. An M.S. (not Ph.D.) student can request an underload in the last semester of study if fewer than 9 credits are required for graduation. For instance, if you have earned 24 graduate credits and only 7 are required to graduate, you can request an underload to register for only 7 credits. An M.S. student with an approved underload is still considered a full-time student.

When applying for an underload with the office of Visa & Immigration Services (http://www.stonybrook.edu/commcms/visa/current_students/parttime_enrollment.html), be sure to use the following contact email for Computer Science: gradadvising@cs.stonybrook.edu

Note that if you request an underload in a certain semester, you must graduate at the end of that semester. Therefore, underloading carries certain risks, especially for foreign students. For instance, failing a course at this stage will prevent the student from graduating and may require reinstatement from the Immigration according to prevalent visa rules. 

A Ph.D. student in the G5 status must register for 9 credits of CSE 699. There is no provision for underload for Ph.D. students --- unlike in the case of M.S. students.