THE FORMATION OF BLACK-HOLES IN LOW-MASS X-RAY BINARIES

We calculate the formation rates of low-mass X-ray binaries with a bla ck hole. Both a semi-analytic and a more detailed model predict format ion rates two orders of magnitude lower than derived from the observat ions. Solution of this conundrum requires either that stars with masse s less than 20 M. can evolve into a black hole, or that stellar wind f rom a member of a binary is accompanied by a much larger loss of angul ar momentum than hitherto assumed.