TENSORS FROM K3 ORIENTIFOLDS

Recently Gimon and Johnson and Dabholkar and Park have constructed typ e I theories on K3 orbifolds. The spectra differ from that of type I o n a smooth K3, having extra tensors. We show that the orbifold theorie s cannot be blown up to smooth K3's, but rather Z(2) orbifold singular ities always remain. Douglas's recent proposal to use D-branes as prob es is useful in understanding the geometry. The Z(2) singularities are of a new type, with a different orientifold projection from those pre viously considered. We also find a new world-sheet consistency conditi on that must be satisfied by orientifold models.