@Incollection{last1989erluterungen,
  author = {Last, Martin and Schütte, Sven and Steenweg, Helge},
  title  = {Erläuterungen zur Karte: "Göttingen um 1400"},

  year   = {1989},
  number = {},
  url    = {},

  const  = {\ text}
}
@Incollection{steenweg1989karte,
  author = {Last, Martin and Schütte, Sven and Steenweg, Helge},
  title  = {Karte: "Göttingen um 1400"},

  year   = {1989},
  number = {},
  url    = {},

  const  = {\ text}
}