  Banados-Teitelboim-Zanelli (BTZ) black holes are constructed by orbifolding
AdS3 geometry by boost transformations of its O(2,2) isometry group. Here we
construct a new class of solutions to AdS3 Einstein gravity, orientifolded BTZ
or O-BTZ for short, which in general, besides the usual BTZ orbifolding,
involve orbifolding (orientifolding) by a Z_2 part of O(2,2) isometry group.
This Z_2 is chosen such that it changes the orientation on AdS3 while keeping
the orientation on its 2D conformal boundary. O-BTZ solutions exhaust all
un-oriented AdS3 black hole solutions, as BTZ black holes constitute all
oriented AdS3 black holes. O-BTZ, similarly to BTZ black holes, are stationary,
axisymmetric asymptotically AdS3 geometries with two asymptotic charges, mass
and angular momentum..
