The circumstances of Pepin's birth remain unclear to modern scholars, especially regarding the legitimacy of his mother's union to Charles. Most Carolingian-era sources dismiss Charles' first union as illegitimate. The contemporary historian Einhard writes merely that Pepin was born to a "concubine", and he does not list him among Charlemagne's legitimate offspring. Although it is possible that Pepin was born to a now-forgotten concubine, Einhard is probably referring to Himiltrude—the first child-bearing partner of Charlemagne, about whom little is now known. However, Einhard and most other Carolingian historians worked in the courts of Charlemagne's successors, and had a vested interest in undermining the legitimacy of the claims of other potential royal lines. These writers may have maligned Charles' union to Himiltrude after the fact, in order to lend a "post facto" justification to Pepin's later disinheritance.