<P> Their infant son Hank Jr. (named after Booth's grandfather) was born off - screen sometime after season 10 and before the beginning of season 11 . </P> <P> Brennan had a brief relationship with FBI Agent Tim Sullivan (Eddie McClintock), also known as Sully, whom she met while on a case when Booth was in therapy due to his grief - induced rage over his self - perceived role in the death of serial killer Howard Epps . Sully asked Dr. Brennan out on a date after their case was completed, and they began a relationship . Their relationship ended, however, when Sully decided to sail a boat down to the Caribbean, and Brennan declined to leave the Jeffersonian to go with him despite Angela trying to persuade her to follow him; psychiatrist Doctor Gordon Wyatt (Stephen Fry) speculated that this was due to Brennan being unable to live a life without purpose . Despite Dr. Wyatt's original perception as to why she stayed, there exist strong indications, especially through Angela's observations of the situation, that the real reason she stayed was because of Booth . Sully returns in Season 12 . </P>

When do brennan and booth start a relationship