Low-mass X-ray binaries with pre-main-sequence companions

In this Letter we examine the idea that a subset of short-period black hole low-mass X-ray binaries could be powered by mass transfer from pre-main-sequence donors. As the star contracts toward the main sequence, the strong magnetic fields operate the magnetic braking, which dissipates the orbital angular momentum, driving the binary to contact. We show that the periods and apparent donor spectral classes of the X-ray binaries with a pre main-sequence donor agree better with the available observations of black hole X-ray binaries than those of binaries with a main-sequence donor. This mechanism also explains, without the need for additional hypotheses, the roughly primordial abundance of Li detected in donor companions of black hole X-ray candidates in our Galaxy.