Let X and Y Banach spaces. Two new properties of operator Banach spaces are
introduced. We call these properties "boundedly closed" and "d-boundedly c
losed", Among other results, we prove the following one. Let U(X, Y) an ope
rator Banach space containing a complemented copy of co. Then we have: 1) I
f U(X, Y) is boundedly closed then Y contains a copy of c(0). 2) If U2(X, Y
) is d-boundedly closed, then X' or Y contains a copy of c(0).