The E15 and E16 centers are typical intrinsic defects in radiation-damaged and semi-insulating SiC. So far, the origins of E15 and E16 have been identified as positively-charged carbon vacancies (V-C(+)) and silicon antisites (Si-C(+)), respectively. However, our complete set of Si-29 hyperfine (HF) data changes these identifications. Our EPR data clearly show that both centers can be well described by V-C(+) but their locations should be different (cubic sites for E15 and hexagonal sites for E16), as recently proposed by other groups. It was also found that both defects have similar high thermal stability over 1000 degreesC. In addition to E15 and E16, we found a new thermally-stable center, labeled HEI1, in n-type radiation-damaged 4H-SiC.