I have only seen Pekins come in white. Those are most likely Khaki Campbells or Chocolate Indian Runners. How do they move? If they have an upright stance and literally run, without waddling, they are runners. If not, they are most-likely Campbells.
Here are URLs to photos of those breeds mentioned:
Khaki Campbell:
http://www.date.hu/info/animaldb/baromfi/kacsa/iv2.jpg
Chocolate Indian Runner:
http://www.dogpix.com/photos/F4poultry/F4poultry-Images/5.jpg
(the one infront is the chocolate)
Those are the most common that they might be, but here are some other possibilities:
Chocolate Muscovy:
http://www.feathersite.com/Poultry/Ducks/Musc/MuscChocF.JPEG
Silky Duck:
http://www.feathersite.com/Poultry/Ducks/Silky/SilkyDuck.JPEG
Possible non-domestic breeds they may be:
African Black Duck:
http://www.feathersite.com/Poultry/NDG/Ducks/AfrBkDuckPr.JPEG
Ruddy Duck:
http://www.feathersite.com/Poultry/NDG/Ducks/Stiff/NARDDrs.JPEG
--Ariel