  For a matched pair of locally compact quantum groups, we construct the double
crossed product as a locally compact quantum group. This construction
generalizes Drinfeld's quantum double construction. We study C*-algebraic
properties of these double crossed products and several links between double
crossed products and bicrossed products. In an appendix, we study the
Radon-Nikodym derivative of a weight under a quantum group action, following
Yamanouchi and obtain, as a corollary, a new characterization of closed quantum
subgroups.
