Black hole candidates with solar mass companions in the orbital period – radial velocity amplitude plane. Solid (dashed) lines show the lower limit on P_orb that, according to Eq. (2), is sufficient to have a 3, 10, or 30 M_⊙ BH in circular orbit with a 3 M_⊙ stellar companion (1 M_⊙ companion). Colored areas indicate where a 3, 10, or 30 R_⊙ stellar companion would overfill its Roche lobe. Candidate BHs from radial velocity measurements: low-mass X-ray binaries (black dots); APOGEE candidate (open circle); LAMOST LB-1 candidate (open diamond); MUSE candidates in NGC 3201 (open squares); HR 6819 candidate (open triangle) and the latest LAMOST selection of giant stars having large ΔV_r and optically invisible companions (crosses), including GS4 (star). More details are given in the text.