Most likely explanation for ULXs?
a 30-50 Msun BH accreting from an O star
                            via Roche-lobe overflow
ß NS or BH accreting from a low-mass star via Roche-lobe overflow (LMXB)
ß NS or BH accreting from a high-mass star via stellar wind (HMXB)
ß NS or BH accreting from a high-mass star via Roche-lobe overflow (ULX  + LMC X-4)
Three classes of X-ray binaries?