martinus was born to byzantine emperor heraclius and empress martina at an unknown date. martinus was declared a "nobilissimus" under heraclius, while the elder brother tiberius was made "caesar" on 4 july 638. according to the byzantine historian nicephorus gregoras, martinus was also made "caesar" on the same day, however the later historian and emperor constantine vii mentions only tiberius. a partially preserved papyrus letter known as sb vi 8986, and cpr xxiii 35, shows that martinus was definitely promoted to "caesar" at some point between 639 and 640, although the exact dating is debated: the german papyrologist who restored sb vi 8986, , and the byzantine historian nicolas gonis argue for a date range between october 639 and september 640, whereas byzantine scholar constantin zuckerman argues for a range between 4 january 639 and 8 november 639.