Facts

[A.] (born 1983) and [B.] (born 1988) were married in 2011 and separated in 2015. They have the common children [C.] (born 2010) and [D.] (born 2013). [A.] also has another son [E.] (born 2017), who lives with him in Germany.

By decision of September 20, 2017, the District Court of Aarau dissolved the marriage of the parties and regulated the consequences of the divorce. As far as still disputed before the Federal Supreme Court, it obliged [A.] to pay [B.] a monthly advance maintenance for the two common children until their majority of CHF 110.-- each (plus any family and training allowances).

[B.] appealed to the High Court of the Canton of Aargau. By upholding the appeal, it obliged [A.] by decision of November 27, 2018, to pay [B.] a monthly advance maintenance for the two children until their majority of CHF 335.-- each (plus any family or training allowances).

[A.] (appellant) appealed to the Federal Supreme Court on January 28, 2019 with a civil law appeal. He requested that the operative part 1 of the contested decision be set aside and reworded as follows: The appellant shall be obliged to pay the respondent a monthly advance maintenance for the common sons [C.] and [D.] until their majority of CHF 110.-- each (plus any family and training allowances).

Considerations

The central point of contention is initially the imputation of a hypothetical income by the lower court. The lower court imputed the appellant a hypothetical income of CHF 1'556.--, which is supposed to correspond to the income of CHF 1'362.-- earned at the company [F. GmbH] for a 100% employment.

The appellant alleges that the lower court falls into arbitrariness when, based on the payslips of the company [F. GmbH] , it assumes a part-time employment with a workload of 87.5%. The lower court in particular overlooks the overtime and public holiday hours worked by the appellant. However, the lower court does not violate federal law when it extrapolates the earnings achieved at the company [F. GmbH] to a 100% employment or considers the achievement of an income of CHF 1'556.-- as…