Any recommendations for an ultralight summer sleeping bag?

PHD or Cumulus ... depending on budget, oh and assuming you want down. I'm assuming you want down as you mention ultralight

Cumulus 250 quilt.
Do a search on hete for it and where to buy it in Germany

ultralight summer bag at 330g - http://www.phdesigns.co.uk/minim-ultra- ... g-k-series

or 260g - http://www.phdesigns.co.uk/racer-down-sleeping-bag